You're not trading a "little extra", you're basically dumbing it down to a 14-15 SEER unit with a "overdrive". A 2 ton 15 SEER unit will have a similar power bill to a 21 SEER 3 ton unit. Both units have similar EERs @ 95 outside. The extra SEER is mostly gained from reduced cycling losses, which go away when the unit is oversized. You're basically paying twice the $$$ to get the reserve capacity. And you will need 3 ton ductwork instead of 2 ton ductwork.
I've never seen a installed 2 stage less than 3 tons, most are 4 or 5 tons.