RE: Sequence of shimming vs nitro
They say run no more than 10% and my .50 has always seemed happy on that. I get lots of flights on a plug so long as I don't spaz the needle setting, but in truth probably no more than 15-20.
MkI, I would say 16% has taken you over the line with stock compression. I've seen a few "Euro compressed" engines go balky at 15%. The power difference between 10% and 16% is nothing to get excited about. I believe your options are straightforward - stick with 10%, or add a shim. The change is subtle, chances are good it would be a trouble-free escapade.
"Shorter" manifold? Shorter than what, did you change that too? Or before?
As a quick and dirty fix you could try dropping the prop load a bit.
MJD