The 2500 rpm engage speed is the speed at which most engines begin to produce useable power. If it engaged below that engine speed the engine would get bogged down and your dead stop acceleration would be terrible.
Most any clutch can be tuned though by the use of different tensioned clutch shoe springs.
Softer springs = a lower engage rpm.
AGK doesn't mess around and would not sell an inferior or poorly thought out product.
They are the engine gurus that worked with sportscarpat and his world record holding Bonneville racer.
You can read all about it on this thread:
Sportsman 200 on Steroids