Simply said, the T-6 is placarded against spins because it does not meet the FAA requirement to recover in 3 turns hands off. That's part 23 coming into play when applying for a TCDS. I've had students and riders both muff up stalls and loops putting me in positions that I normally wouldn't go in to. It's a very honest spinner both inverted and upright. If its inverted it quickly returns to a normal spin (in about 3/4 turn) then its standard recovery (power, rudder and stick simultaneously, wait, and recover). The biggest issue I've seen is the engine sputtering a little at lower power settings due to it being a float carb.