Tested, felt the same result... after entering the snap roll, it develops a spin, even if inmediate exit procedure is applied at the moment to enter the snap roll.
It also happens with Bf-109 when full deflections are made... but not if partial deflections of the controls are done, I say that because when I'm in combat, sometimes I accidentally stall, but I can stop the accidental snap roll in a half roll... that's because I was not pulling the stick to the end
Anyway, I'm not sure if it's realistic or not for a plane like that, the only real snap roll I've seen is one from an I-16 with only two turns and a precise stop.
That is hard to evaluate... is there any Spitfire pilot here?