A sealed room with temp control, humidity control and CO2 enrishment will outperform a room that doesn't have it, all other variables being equal. Trouble is, all the other variables can't be equal to have a fully optimized CO2 setup. Why? Because CO2 enriched plants like warmer temps and higher humidity. They're also going to drink more and use more nutrients.
If I were in your shoes, I would stick to open venting until you can afford to do the sealed room thing right.
So you optimize the rest of the variables to maximize the effect of the CO2. Water more often, feed more frequently, raise the temps a bit (I never go over 85 F and prefer to keep things right at 80 F at 1500 PPM), set your RH where you feel is best for your setup, etc. I don't understand the problem.
Adding Co2 doesn't have to be a huge investment, and depending on equipment used, won't increase your energy bill by any significant amount.
Using a water cooled burner will prevent or greatly reduce the need for any additional cooling depending on the temp and rate of flow of the water being used. Run to waste is inexpensive compared to setting up a closed loop chiller cooled system. Chillers are where water cooling can get expensive. My gas comes out of the burner at 79 degrees (with the optional Ice Cap form Hydro Innovations).
However you choose to do the setup, it sounds like you are in a perfect position to run the two rooms and do the side by side comparison yourself.
I've been running enriched, sealed rooms for about two years now. It is no more difficult to manage my garden now than it was when I was setup with a fresh air intake and exhaust system.
Good luck whatever you decide to do!
