Use the money you would have spent on a custom tune to put towards a wideband o2. I can not emphesize enough what that peace of mind is worth knowing you aren't running lean. Then if you have problems with the KB tune you will know and can let them make adjustments to it so that you maintain your warranty.
BTW, I run a copy of KB's tune with my saleen sc and the tune is solid. I had to make very few adjustments to it. Now it is a little conservative though, their tune commands 11.0 a/f ratio at WOT, so you'd could probably get by with and make a bit more power running 11.5 or 12.