Any problems adding a new SSD to iMac 2011?
The German IT magazine computerbase has done tests comparing the efficiency of  TRIM and garbage collection in SSDs of 2010.

The result is that recent SSD have a very effecient garbage collection so that the performace difference to TRIM vanishes. (source, German)

Upgrading the HDD on a Macbook does not void the warrenty (source). However, it does on a Macbook Pro (I think). Since replacing the hard drive on an iMac will require lots of tinkering, I'm pretty sure that you will void your warrenty.

Detailed guides as to how you replace your harddrive can be found here:
They include pictures, step-by-step guide and info on what kind of screwdrivers you need.