I regularly drive a 99 Mustang Cobra with about 345hp. It has an IRS and an limited slip. Tires are only 275s in the rear. It spins the tires a lot easier than my BDR.
Something with there being very little weight to push down the road, these tires just grab and go.
Food for thought on Limited Slip, which typically I do favor and recommend, I just haven't seen the need in the BDR:
http://www.youtube.com/watch?v=AHmSDXrVG84
With that, I'll shut up now. Good luck Kevin, will be interested in hearing your view whichever choice you make once you have keys in your hand.