Actually, with the Mac Minis the reason for the 5400RPM drive is to keep the unit cooler. A 7200RPM drive generates more heat and in such a small case with a small fan heat is difficult to dissipate. The fan would be running hard a lot of the time. In the laptops it is as much about battery life as heat.
The Mac Mini isn't that hard to upgrade, you just have to go slow and careful. I upgraded a mixture of 32 of the 2009 and 2010 model Mac Minis a few months ago with SSDs. Dramatically improved overall performance as would be expected.
I haven't got the money now but I have applecare until august 2014 so will leave it until then as otherwise I invalidate the warranty anyway as ram is the only user upgradeable part