Hi!
I am the Swedish importer of MVVS engines (for more than 10 years) and I have been active in pylonracing for many years using these engines.
The Silencer you have bought isn't going to work properly as it only functions up to around 15000 rpm and the MVVS Q-500 engine develops its power alittle higher up at around 16000-19000rpm.
Instead you should get the No 3246 minipipe. This is made to work from around 15000 to 24000rpm.

Best prop if you want to go fast with this engine is a 9x6.5, 9x7, 9x6 or a 8.75x7 APC prop.
If you run FAI fuel (80/20 castor based fuel) then use a Rossi 6 glowplug (rather cold plug), it will give you more performance than lets say a OS 8 or Enya 3 sport glowplug.

It is very common here in Sweden to run 10-15% all synthetic oil in the fuel...it must be in France to I suspect?!
I run 15% Motul "Micro" all synthetic oil (French racing oil brand) in all of my engines for several years with total satisfaction...engines seem to run and last much better than with castor oil.
However I have not run one of my MVVS Q-500 engines with this oil so I can't say for certain it will work. But I have flown my Q-500 racers powerd by a Webra .40 GT and a MVVS .40 with standard silencer on this fuel and they have certainly run good for several years.

Regards!
Jan Karlsson
Sweden
http://hem.passagen.se/airracing