raideron -- there are no flat-bottom winged Nexstars -- I suspect that you are being misled by the add-on anti-stall extensions, which do provide a flat bottomed profile in the outer wing. These are removable. The wing also sits with slight positive incidence, which makes the flatter aft run of the wing underside appear to be parallel to the chord-line -- it isn't. If you doubt me, call Hobbico -- they think that the wing is a semi-sym.

I do agree that a stock Nexstar can't match an Avistar in aerobatics, but the Nexstar is also a much larger & heavier plane, and being so, it has tons of potential for modification. It really flies well with a bit of hopping up & a good dose of power.