BFG 18 inch g force drag radials improve traction everywhere. There's barely any sidewall, so handling really isn't effected.
As far as the original post, IMO a new supercharged saleen is too much money.
I'm told that if you have a regular 4.6 mustang, and you have an authorized saleen installer put the supercharger on, you keep the factory warranty. This coming from a guy that installs them all the time. He is NOT certified yet, but is scheduled to goto saleen for certifications.
That could save you about 5000-6000 dollars.
I'd get a used 05 saleen mustang, then put the supercharger on it. Figure on $8500 for supercharger and install.