One more idea if you just want a weekend bruiser that will give you gobs of power: the 2013 Viper. It will apparently sticker a bit above $100K and its interior is STUNNING -- plus you get that glorious 8.4L V10 putting out 650 naturally aspirated horses and 650 lb-ft, and of course the proper transmission. I agree with the above post about the R8 possibly being a great (and different) experience from the M5, but the speeds would indeed be similar, whereas the Viper would be an even greater departure from the M5 experience-wise, and be faster, AND cost less than most of the other cars mentioned here.
There's only so much longer that you'll be able to get a naturally aspirated engine, and unless you want to step all the way up to the 599 or Aventador, this is as big and brawny as NA is going to get before it disappears, probably (and sadly) for good.
