So..........I got a good deal on a 07 Black XLR($54K-200mi. Demo). I ordered all
the parts necessary to put the V rear sway bar on. Also Caliper brackets for
Corvette Z51 rotors(calipers are the same). I have the Z51 rotors left over from my Corvette. I tried my corvette front race wheels and tires on the XLR front and rear.. Fit
is good with no rubbing, including front, lock to lock.(18x9.5 53mm offset with
275/35/18s). So lots of room. However, the XLR will need taller tires.
So........the big question!! How short or unbalanced can I get away with??

I know 255/45/18s on Cray Scorpian wheels(9x18 50mm offset will work.
however, I would perfer 275/40/18s on 9.5x18 wheels. or a stagger like
255/45/18 on 8.5x18 and 275/45/18 on 9.5x18. 19 in wheels are out as either
the tire options are too unbalanced or you have to use Parelli runflats which
have very poor ratings. Yes I want runflats! So any info would be appreciated.

Sounds like a couple of good options. I've been contemplating wheels myself - let us know what you come up with!