Shh, don't let the secret out.
Last time I tried it with my A6M5b I hit somewhere in the 600mph area(or was it 570... anyways!) before shedding parts(according to film). Not sure if this has changed since they fixed the bugs in the CT for the A6M. That and I haven't had the initiative to climb close to 30,000ft again. Also note even though I wasn't shedding parts till extremely high speeds I was far in the red as far as compressing goes.