4x4 I run 2 line arrays @ 300w per array... I'd probably stuff the same thing in the 3x3 too....
With the COB tech, I get higher yield that what I was getting with the blurple, plants get bigger and stretch more...and sticky... wow!
No LED is going to beat out a 1K HID.. not there yet. What lamp is in your 3x3?? I'd probably start with that and do 2 300 watt fixtures and see how it works for you. You can do a side by side too and see if its worth it to switch.
You're going to get more light with HID .. for sure. There's no shortcut and I don't care what anyone says, HID is still the best bang for the buck.
The only reason I switched was due to us going away on vacation... with automated watering & lighting. I downsize a little as far as plant counts but the lighting stays the same. The heat in the summer time kinda made me nervous with the HID. I did one run automated with HID and worried the whole time we were away. Get a runaway plant (I've had that happen more than once) grows into the lamp.
Here's where you can save, if the 3x3 you were running say a 1K lamp and switched to 2 300w fixtures you will save some electricity cost and of course heat.
Would you do as good as the HID... probably not, but it will be close.