Shift kits don't change the rpm of the shift, what they do is "firm" up the shift. On an automatic, the factory will "slip" the transmission between gears to make the shifts smooth. Ie, they'll gradually release pressure from 1st gear clutch/band while applying pressure for 2nd gear. This gives a smooth almost unnoticeable shift. However the clutch packs/bands for both gears are slipping which wears soft parts and creates heat. When you start adding more power, this gets worse. So to keep from burning up transmissions, you have to not slip it as much, which means decreasing the amount of time for the shift. The drawback is that the shifts are very firm. If the shift is firm for truck, it will be very firm for race. Drilling holes, swapping springs, etc all increases pressure applied to the clutch, or decreases the amount of time the pressure is built up. With newer solenoid (electronic) controlled transmissions a lot of this can be done in the computer, but you still have to deliver the fluid which is why the holes are drilled, etc.
Cal: This is why I want to find a good "core" AW4, so I can do this. Not sure if I'll use raybestos clutches, but I do want a fresh trans with firmer shifting.
When I did my Honda Odyssey transmission I used an old wheel bearing race and cut "window" in it so I could pull the clutches apart in that. That was an interesting transmission. Essentially a manual transmission with clutch packs, and no planetary gear sets.