I had the old AP Tourby, the movement is really well done as a Tourbillon but I had it fully torn down and serviced. It measured 12.2mm. My watchmaker believed he could easily get .3mm off it .5mm at a push and .7mm at the breaking points of reality. For that little off I decided it was not worth it. My normal JF 15400 V2 was 11.2 and he took .6mm off and could maybe go another .2mm but with a lot of work and care. I did think getting the V2 down to 10.6mm was worth it but getting the tourby down to 11.7mm when the gen is 9mm was not worth it in my opinion. Maybe this one is different but I doubt it.