Hi!
I think you use too much nitro and set the engine too lean! And how about the lengy of that exhaust tube...seems very long too me.

Try 80/20 fuel and a Rossi 3,4,5 or 6 glowplug and see what happens.The Rossi glowplugs are some of the best plugs available (Webra use the Rossi 3 as standard plug on their Webra .40GT engines but call it a Webra plug... ).
Rossi 6 was the plug of choice over here when we flew Sport 40 pylon in the nineties using MVVS , Irvine and Webra engines. Props used were APC 9x7, 9x6,5 and 9x6...we also tried some of the APC 8" pylonracing props with no better performance. My MVVS Q-500 6,5cc engine running a Rossi 6 plug, 80/20 fuel turned a 9x7 at around 17000-175000rpm and a 9x6 APC at 18500 on the same plug.

Are you sure the webra engines are set up to run on 10% nitro? An OS 8 plug is definitely too hot when running on a pipe! Try a MVVS minipipe( short or long).40pipe and see if that will improve things.

When I raced those engines (MVVS and Irvine) I had a fairly broad range and setting them were no problem (we used 6oz Tettra "bubbleless" tanks).
Using the pinch metod isn't the best metode of setting a racing engine...don't know any racer that uses that metode.
Best way is to learn setting engines by ear!