Not having a lid over the bucket will certainly aid heat loss, in the same sense that a covered pot of water will always boil faster than an uncovered pot. And it's true that many people feel that open fermentations (and fermenter geometry in general) can have a drastic effect on yeast-derived phenols/esters. But your theory about the escaping CO2 doesn't hold up, since the CO2 will leave the bucket either through an airlock or through the open top. You aren't really building up pressure in the bucket unless the airlock is clogged; to get through one inch of water in an airlock will only create 0.04 psi of backpressure.
In reality I doubt you'll notice a difference in your temperature this way, but like I said any difference will just be due to the lack of a lid. I'd recommend you get a little more active with your cooling; a swamp cooler can work wonders, especially when you couple it with some ice. And you're just $20 (for an ebay temp controller) and an old minifridge away from a "proper" fermentation temperature.