You mean you don't play in psychedelic mode? You don't know what you're missing!
To be honest though, screenshots don't do the difference justice, it's hard to pick out - Only when you are actually moving around do the mipmap borders start to show (as I'm sure anyone like myself who used to own a Radeon 8500 and was forced to live with bilinear with AF will tell you!).
Hmmm... Well I don't know if it's worth the performance difference or not since I obviously can't tell for myself (Have a GeForce FX 5900 Ultra)... But I used to use 16x Performance ansio on my Radeon 9700 Pro and have now treated myself to 8X Quality ansio on my GeForce FX 5900 Ultra and the difference in quality is pretty amazing, so I wouldn't be surprised if there is a nice bit of extra eye candy....