I don't know if it would have or not, but what it WOULD have done is opened the game up to a wider audience, which cannot hurt a game.
That's true, certainly, and it is a problem that's really plaguing the game industri at the moment, because quite frankly, the latest and greatest tech in use right now just doesen't scale well, they rely on tricks that make things look deeper and more detailed than it really is, and thats great if you have the vid-card power to use it, but if you don't, many modern games actually look worse than older games if you disable shaders and other advanced features like that.
Quite frankly, i feel the industri would be much better off if they put less emphasis on the graphics and used that time better, making more polished and deeper games, atleast for the PC market, and i'm not alone in thinking that, some of the best selling games on the PC have pretty lax requirements, so obviously it's not bad for buisness.
But that said, i don't think UT3 would have had much more success had it had more lax requirements, a bit more probably, but the fact that it's out for Console means it was bound to take a big hit on the PC, alot of gamers don't have a top of the line gaming PC because they decided to buy a Console instead of upgrading (a decition i can't really fault them for, since most games thease days are cross platform), so lots of people have just picked it up for their PS3 or 360 instead, where they are garuenteed to run it at max graphics, and whilst that does line Epic's pockets, it does nothing to help the online numbers on the PC.
Thats just one more reason i don't like cross platform development, it splits communities
But probably the biggest problem for UT3 and performance is not if or not you can run the thing, its that it doesen't play well at all unless you have a stable high FPS, if you're FPS are all over the place (even if they do stay above 30), the game handles very poorly, the mouse just wont be reliable, and you need good aim in UT as much as you need a full health bar and a good gun.