If you could exhaust outside would probably help quite a bit. Depending on your climate, there may be some benefits to drawing outside air as well.
I don’t really have the time to break out a fancy drawing tool, but let me see if I can explain what I’m thinking… How about an exhaust header that has 2 separate draws from the main header, one going to each hood, and then exhausted to outside? From there you would essentially be exhausting your rooms and cooling the lights separately from a single fan.
You would want to install a damper between each hood and the main header for fine tuning. Otherwise, as long as the fan was sized for the job it should work. The only real downfall would be if you were using carbon filters, you would need more than one. I think the added benefits would be worth it personally. But you have to make it work for you, so maybe if nothing else I sparked at least an idea.
Personally I think led is much easier to grow with, but that could be an opinion based on my personal experiences. To get an led fixture that’s truly competitive with the 600 will cost some cash though. But you’d also be ditching the cooling circuit for something that will simply be responsible for air exchange.