I was actually very close to buying one in auction last month, been looking at these for a while. From what I recall, the performance pack *plus* utilises the forged pistons from the then new SLS AMG, a lightweight crankshaft, new connecting rods and a new engine management system, along with LSD and other carbon goodies. I've driven quite a few of them (standard, performance pack, straight through pipes etc) and they are an absolute blast to drive, I've not driven many cars that rival the roar of that 6.2 litre V8 - and I drive for a living!
Although the interior may be dated, Id be looking at a later facelift example, 2012-2014, not only do they look meaner, interior is slightly improved and the gearboxes were also revised post 2011. I don't think there will ever be a version of these engines on the production line again, and its a real shame. If you have some cash to splash, you wont regret spending on a good example of a C63. Just bare in mind, most of these have had a hard life for obvious reasons, so would be best to experience it first hand before taking the plunge - but as long as its been weel looked after, doesn't mean you cant join in on the fun