I use the Donny Fl Sumo and FatBoy on my Taipan Veteran .22, and 'prefer' the fatboy look, the shorter squatter end can, and it's pretty quite - maybe not as much as Sumo but then the Veteran is pretty quiet as it comes from the factory with the included shroud.
I recently spoke with Donny about the Sumo - like you I was thinking of getting a .25 and wondered if I needed to bore it out. Donny said his adaptor - for my Taipan, at least -- the adaptor is 8.2 mm inside diameter/bore, and the Sumo bore is 8.2 mm as well - which is just fine for the .25. This translates as you know to .322 caliber.
From what I read on silencer build forums, adding .060 inches to the projectile size and....it works, for a .25 seems you need .31" bore and we have that with the Sumo.
As you might be able to tell, I've been over this one myself intensively!