Resprayed the muffler with exhaust paint and refitted it after a "race" down to Halfords in holiday traffic for some high temp gasket goo. A quick run up of the engine and the awful bark at the top end of throttle appears to have gone. As I am at home I could only give it some quick bursts of throttle and so unable to verify the top rpm although the idle is nice and steady. Looking forward to getting up to Cashmoor next week to give it a flight.
On a spur of the moment I rushed up to Cashmoor on Saturday afternoon to give the Extra a flight with the modified GP38 muffler and maybe a tweak on the jets. The exhaust note now has lost it's harsh bark sound and is more akin to a DLE muffler. The transition issue, dying or cutting out, from low to high on the throttle has gone which I had when the OS muffler was fitted. I did not for the first flight adjust any jets. Prior to the first flight I checked the RPMs. 1850 idle and WOT 7550. Now it performs. Prior to second flight I leaned the low just a smidgeon but still a bit rich (engine is still fairly new) and it flies extremely well. Happy now! Now maybe I can get on with maintenance on other stuff.