Oh man, you just can't seem to get a break with this thing!
Normally it would be as simple as a trip back to the shop with an irate look on your face, but with the problems you've had with this shop, I'm not sure that would do you any good at this point...
With regards to the characteristics of the shift kit, you should see relatively the same shifting as without the SK when accelerating 'normally' (i.e. steady, gradual accelerator application), except for a
slightly firmer shift on the 2-3. When accelerating hard from a stop, 1-2 and 2-3 will be quite firm, with enough of a slight negative g-force effect to just start your back away from the seat before the shift is complete; but NOT a clunk or a slam into gear. Downshifts at speed should feel just about the same as without the SK.
From your description of the downshift trouble you describe, I suspect a valve body problem. The SK install instructions as pretty idiot proof (there are even pictures), although it's possible they missed a spring or checkball somewhere, or as someone else said, they might have installed the blue #2 accumlator spring instead of using a stock spring. It could also be as simple as them not properly re-torqueing the valve body bolts several times.
You might try taking it back to them yet again, but this time bring a filled-in small claims court complaint with you and demand satisfaction within say 7 days or you'll sue them (and be ready to follow through). I recommend bringing the car to them all warmed up, and make the manager go on a 20-30 minute drive with you to demonstrate the problems.
