Heh, surprised to hear anything but 9psi when talking about KB's.
Just go by their website. the 2.8L is the "competition" blower. that means non-stock internals. Get the 9# 2.1L set up. Then down the road put in a cobra crank and some forged pistons/rods and up the boost to 12#, or sell the small blower and buy the bigger one. Depends on how much money you have.
Also, all my life I've heard that 4.10's are terrible for a blown application. 3.73's are the most popular among the s/c crowd, I believe. If you call/email/research KB, they'll tell you from the testing they've done (and are still doing, btw, on just about every mustang since the 5.0's), with the 4.6L and their blower, 3.73's are the perfect gear for the 1/4 mile. So you might want to rethink the 4.10's when the time comes.