For your goals, you will definately need to build the internals. How far is up to you. From my level of knowledge about the KA (admittedly less than the SR), I would think you could get away with just pistons. You will lose the rev-happiness of the SR but will gain torque.
It makes the same power at 8 psi as the SR does at 12.
You can't make that claim that broadly since there are a lot more variables that go into that. Scott's stock-bottom SR was running about the same boost pressure as Ryan's built KA-T and Scott's car was making about 140 more HP (granted, Scott's run was on race gas).
Anyways, the torque is a huge difference on the street, but will make it a bit harder to keep from blowing the tires off at the strip. Even with a slight compression drop, the KA-T will have a bit better response (less lag, better off-boost, etc) than the SR. It will also be marginally heavier up front.
Either way, don't think of it in terms of SR/KA-T. Think of how you want your car, do you want to take your current SR and exaggerate it's powerband from 4,000rpm up, or do you want a car that is just torquey as a mofo (kind of like my dad's 951).