When I spoke with the Sako representatives in this country, they told me that the TRG22 has a chrome barrel and chamber. Any time you cut a barrel with a chrome bore, you run the risk that the chrome in the barrel will not cut cleanly, and will chip. Obviously, getting a chip out of the chrome near the muzzle would be devastating to the accuracy of the rifle.
For that reason alone, if you want a 20 inch TRG, buy that barrel length, and live with the Sako thread pattern (18x1mm IIRC). It pretty easy to find someone who will make a muzzle brake, or suppressor in that thread pattern if you don't want to pay for a Sako brake. If you want more versatility, you could always get a thread adapter going from the TRG pattern to the 5/8 X 24 thread pattern that you want.
I would be terrified about having a barrel with a chrome bore cut. Although there are plenty of people who have done so successfully, I just wouldn't want to take the risk.