I don't know about that, not long ago I had a 12"x24" dark room vent (similar to your vent) for my intake and a 6" Vortex for the exhaust and it worked well with a 400w setup but the temps went into the high 80's (from the low to mid 70's) when I upgraded the light setup to 2 x 1000's. I changed out the intake dark room vent for the speed controlled Vortex intake fan and the temps came back down to the mid to high 70's. I think you might be surprised at the control it gives you as long as your exhaust is sufficient. An active intake will actually make it easier for your exhaust fan, the fan doesn't have to pull/suck (against that vent) so hard. I could hear a difference with my exhaust fan when I installed the active intake, significantly quieter.
I've thought about using an air conditioning unit and doing a closed room also.