I have actually tested the card, on air, water, tec and in a freezer at -40.
The HSF linked to will not improve the clock speed of fermi IMO, they will only reduce the noise levels and dump a lot of heat into the case. If spending £40-50 on voiding your brand new GTX480 warranty for ZERO performance gain seem like a good idea then that might be the way to go. Although I'm not sure why anybody would do this when you can get all that under warranty £210.
I my experience, if you want BIG performance out of Fermi ( The oly point the architecture doesn't fail horribly on ) you need to lower the temp dramatically as you are dealing with a BIG, HOT, INEFFICIENT 400~watt card. To deal with this I ( me, not some guy on Youtube ) have found you need very good water cooling, and even that is sometimes not enough to bring the temps down. Strap on a 400~ watt TEC and and we found you start to find some head room to work with. If its all worth it I'm not sure, but it will let you push the card that little bit more.
IMO and most other people I know you should run the GTX480 stock or go all out and push for 900+. Paying £40-50 to void the warranty of a brand new GTX480 for the same performance just seems dumb to me so maybe I don't have a clue, as its all seems clueless to me, but hay ho I submit as some people here seem to have masses of experience in such exercises...
Had your ears tested lately ?