Some planes are just easier to get out of the inverted flat spin, others aren't. It's an issue that mostly has to do with combat trim, and for a steady reversal at very low speeds, during the vertical, combat trim should be turned off prior to going vertical.
The Corsair, P-51, Ta152, Fw190, 109s etc.. many planes have this issues. Usually, if one doesn't manually trim those planes, and go vertical and try to reverse under 100mph speeds, they'll fall into that inverted flat spin.
But of course, some planes, like the Spit9, N1K2-J or the La-5FN, La-7, rarely encounters this same problem. I think I actually did an inverted flat spin in a Spit9 way back, but I pulled out of it in about two seconds - "whoa, I'm falling on my back.. Ok.. calm down.. use rudders.. attempt to nose down first.. oh.. I'm recovered already.."
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Anyway, I agree with Bozon. If the Spit14 needs those 60 perks, then the G-10 needs 50 perks, and the La-7 needs 70 perks.
If the G-10 and the La-7 is free, then by golly the Spit14 needs to be free, or if it really has overuse issues, lightly perked around 5~8 points.