OK I can see Im carrying on my own thread here, no-one seems to know or wants to share their experience/knowledge. So Ill post my results here so maybe other might try my solution.

After 2 trips to the field and several people having a go at tuning this OS50SX without much success the final idea was to try a smaller prop. With the APC11x6 it was clearly unloading and reaching a peak in a shallow dive but anything it else it was a pig.
I tried a Bolly 10.5X6 prop and got better results but somewhat slower in flight. Today I tried an APC10x7 and it screamed like it did new with the 11x6. Defineately a HUGE improvement in performance. Why ? Other than a smaller prop so less load, I dont know why it dropped performance from its original performance with the 11x6 but at least its flying great again. Compression is excellant for a ringed engine so its not that, carb is clean, needle is not blocked although one time the guy tuning it was able to almost completely remove the needle and NO change in revs, weird we know but everything else checked out OK.

Oh and my testing indicates that performance with the cowl on is much better than off.