The GRP's, both the 21 and 28, have had tuning issues. On some that were returned there was air leaking from the exhaust port going around to the side bypass chamber on the sleeve. Low RPM tuning provided frustrating results, newer versions that had the smaller restrictors seem better - 'read': better tolerance on the maching of the block.
P3's would offen advance the timing too much but would definitely improve performance. The stock plugs are simply too cold for the engine. P4's were a better match, however the ease of tuning with a standard head button AKA GO or Novarossi, made the engine run like a mid range Novarossi - which is what it basically is.
Although our local track has been great in supporting them, I'm going back to the diehard long live Novarossi's. Even Truhe and Drake had inconsintent Flameouts which are not as frequent now. In another year I think Manufacturing will have tighter tolerances now that initial demand has weaned a bit.

I'm not meaning to start an arguement, just stating my experience.