Mongol,

Are you sure your Osmose is under 5Kg? My first Osmose equiped with the 160 had unlimited vertical at 2000Ft field elevation, at sea level is rare when you need full power from the 160.
When you said it is not a capable performer, well, it ended 2nd place at the last worlds!!!!!
You should try a 17x13 instead of the 18, you might be loading the engine so much I think it might be accelerating kind of slow in flight, and the OS 140 is a great engine, but is really a toy compared to the performance of the YS.
Change to the YS 160 and it will be a completly different aircraft, start with APC 17x12 and 30% nitro, after 6 gallons of fuel try the 17x13 if the engine is new.
If you are not running high nitro on the OS it will never give the required performance, and remember the OS 200 is a sport engine not intended for precision aerobatics power . And as John just said there is no comparison in power between your OS model and the YS.

Regards