Thats not true. Whilst the camera angles are fixed, the camera itself does not follow a set path. =Tt doesn't give them any room to really optimise at all vs a free camera. The lack of free visual control is more a reflection on the fact that the game has heavy use of the face buttons than anything to do with optimisation.
Quill I'm playing GoW3 right now and that camera's on rails alright! Firm rubber ones I'll admit, so that if I push the stick waaay over to the side it slightly 'blips' beyond the set FOV, but rails nonetheless.
It's a great engine, but whether they could or couldn't allow free camera they haven't, and that gives huge advantages in how much you can shove on the screen as the Santa Monica devs know pretty well, if not near enough exactly, what you are capable of putting on screen at any time and can control the streaming of assets/textures/geometry accordingly.
A game with full 360 view in your control simply has account for less predictable control of what you're going to choose to put on the screen.
GoW3 is also focused on character models (understandably) with very static geometry (even the big moving ones) an minimal phyics so far as I can see from me play experience - which is fine, but again less versitle so far as the evidence seems to show. And I like versitily and control of the camera - so there!
Besides, if we're just talking pure visuals then GT5:P - and almost certainly GT5 from the media I've seen - are the real visual masters on PS3 in terms of the produced image on screen.
Try to be reasonable... its easier than you think...