I will try to give you the best info I can. As far as your heat issue, i see you have a vented hood, are going to be venting it with a blower/fan? Or are you already using a blower/fan and still having issues? The reason I ask is I didn't see you mentioning it and so if you were NOT venting and having heat issues then I would suggest first looking into a fan for heat extraction.
Now if you already have a fan hooked up, where does it leave the tent and where does the air go? Meaning, does the hot air exit the tent and then blow directly into the room that the tent is in? or do you have the ducting pushing the hot air outside the room your tent is in? the reason I ask this is because I have my tents venting into the room the tents are in, and over time as the lights are on, the heat of the room raises, therefore also raising the temperature of the tent. For me since I was unable to directly vent outside, to minimize heat I have to keep a box fan running in my window blowing outwards 24/7.
The reason I am mentioning the venting first as it would be the first "link in the chain" of addressing for cooling your grow area.
Now as far as adding extra light and reducing the wattage of the HID lights by dimming them and would it help... thats a tricky question..
Color shift:
With HPS, when you dim a bulb lower than the max rating, not only does the intensity obviously change, but so does the spectrum, and with HPS the more you dim it, the more it shifts from orange/red to yellow. Now as far as plants needs and how they take in light, yellow light is used, but not as efficiently as orange/red light, therefore you would be dimming your ballast to reduce power, but also reducing the actual efficiency of the spectrum by shifting it towards a lower absorbing color. This is not a huge shift but it is something worth mentioning.
Efficiency
Now I am definitely not as knowledgeable about the technical details of HID as I am with LED, but from what I have read, by dimming the HPS bulbs down, not only are you changing the spectrum, but also lower the electrical efficiency in converting watts to light. Now, like I said, I have not heavily researched HID for this specific question, but the general consensus I have gathered says if its a 1000w, run it at 1000w, if you want it run lower, you would be better off getting a 600w ballast and 600w bulb or 400w ballast and 400w bulb as from what I read this is where HID lighting runs most efficient (at max power). For example lets say that at 100% power draw the HPS puts out 100% of its lumens, when dimmed to 75% power, the HPS is only putting out 65-75% of its total output, and at 50% power draw, HPS is only putting out 35-45% of its lumens. (information based of Osram/sylvania data sheets).
Therefore the efficiency does go down, not drastically but its something to also take note.
Another example would be:
a 1000w HPS puts out roughly 140,000 lumens at full 140lm/watt (1000W)
a 1000w HPS puts out roughly 91,000-105,000 lumens at 75% (750W) or 121lm/w - 140lm/w
a 1000w HPS puts out roughly 49,000-63,000 lumens at 50% (500w) or 98lm/w - 126lm/w
A 600w HPS running at 100% puts out roughly 92,000 lumens. or 153lm/watt
*these numbers are base line data for example purpose but each bulb would have slightly different values, but the trend would be roughly similar.
Bulb Life/Lumen Maintenance
From everything I read, by dimming the HID bulbs down it also has a great potential to lower the life of the bulb much sooner than the projected lifetime which can range from 12,000 hrs to 24,000 before replacement. I couldn't find the actual difference data quickly searching but I would just take this into consideration as well.
So, by what I just said, basically if you decide to dim your HPS, you are losing efficiency, bulb life and slightly altering your spectrum. Now will this help you cool your grow area..
Heat
Ive done quite a bit of reading on this topic and the general statement "LED's run cooler" is only half true. According to physics laws, any electric lighting system in a closed environment despite its electrical efficiency at equal wattage will produce the same heat load to the closed environment. What this means is if you had 10x 100w bulbs, 1x 1000 bulb, 1x 600wHPS + 1x 400w LED would all add exactly the same heat load to a sealed environment. (not considering air venting and heat extraction fans). How this breaks down is 1w=3.41 BTU/hour. Now many would say, but wait, isn't a light that is more efficient going to put out less heat...and the answer is no... because when light hits a surface it is reflected and/or absorbed. Eventually all light particles (photons) will become absorbed which is why when you turn off a lightswitch, the room goes immediately dark, because the photon energy is absorbed into the surroundings. When a photon is absorbed, it puts off heat, so despite having 2 lamps with different efficiency, the total heat added to the closed area will still be the same regardless of the light source as heat load is constant with draw wattage as the formula above points out.
Now why did I mention this? Well if you were to lets say dim your ballast down to 75% and then add 250W of LED, the wattage draw would be the same, therefore the heat load would be the same. Yes LED light is absorbed into the plant much more efficiently than HPS so the (250w of energy) would be used by the plant more efficiently so yes you might get a slightly better yield by adding LED, and the buds definitely would gain quality as LED is more targeted to the specific wavelengths plants absorb and utilize, however in the topic of "will this cool my tent" the answer would be no...
I've found that HID with air cooled hoods vs LED of same wattage actually can be slightly cooler than LED due to air cooled HID having a way to evacuate the heat directly out of the grow area, where LED actually vents the warm air right into your grow area....
So all in all, I would say if getting a Mars II is within your budget, give it a try and I definitely think your quality may go up a little, as well as from other comparison tests usually led does produce more THC and better flavor as well. In terms of heat, I don't think it really would help much as far as reduction. By dimming your ballast to 75% as the example above shows, the efficiency loss isn't that much, so by adding a Light with more efficiency in photons being absorbed (led) I think you may even see a tiny yield increase.
I hope this helps, answering it was actually quite fun for me because I don't study HID lighting much anymore so it was not only refreshing to re research it, but also learned a little myself
If you need anything else, feel free to ask.
It thrills me to find someone who gets the same rush from sharing research as I do. 
