It can affect frame rates if the game tried to load environmental items while the game is running, like in Oblivion, if I walk into a new sector and it has to load something the frames can drop momentarily then recover. This has since been eliminated since I got my Raptors but on my old Seagates I noticed it for sure.
Frame rates and loading time are two completely different things. Frame rates is done solely by your video card while loading times are done by your hard drive. I don't think you'll see much of an increase from an 8MB cache to 16BM cache. You'll see the difference though from a 7200rpm drive to a Raptor though.
Your frame rate wont increase directly but the data from the harddrive to memory will increase. This removes a bottleneck in the system as most everything is grabbed from RAM.