I am an XJ 4.0 man myself, but we get the HEMI 5.7 and 6.1 Grand Cherokees in the shop all the time and they are very impressive to say the least. In fact... we see quite a few of them on the drag strips in the MOPAR competitions. There is a ton of aftermarket support for these engines that will take them from small bolt-on 20hp - 40hp gains, all the way through supercharger applications. We are still in R&D mode for the 2011 -2012 models, but the 2010 and back are a wide open playing field.
You can pretty much get them to perform however you see fit, based on your wants and needs.