I used the FMF silencer instead of the AAEN silencer that matches my chamber because I read in one of your old posts that the AAEN silencers were causing pistons to seize.
I'll chop up the FMF silencer and replace the perforated tube with one of a larger internal diameter. Is there any size that you would recommend? I realise that you're not going to be able to say the exact size to achieve the peak power out of the chamber but what do you think would be a safe size to use?

It was their spark arrestor design that was torching pistons.  Their spark arrestors were adding too much back pressure.  A little too much back pressure will add a lot of heat to the piston crown without hurting the power.  If you get way too much back pressure the engine cannot make enough power to make enough heat to hurt itself.  You can increase the back pressure to a critical level by making the core too small,  the stinger too small, a spark arrestor that is too restrictive, a stinger that is too long for the diameter of the stinger, there is an infinite numbers of ways to get too much back pressure. 

Getting the back pressure right requires a lot of testing and usually requires some pistons to be sacrificed to find the right combination of stinger and silencer for the exhaust system that it is attached to.