My cousin has 2 GRP Tuned 21's. they were both run in the same. they have both run the same amount of fuel. they run like 2 different motors.
One of them you would swear could move a house. the other is a pig, it would be luck to pull its own weight.
Alot of people have seemed to have the same problem with these motors. You can get a good one or a bad one. So if you go this direction hope to get a good one
I have 2 RB's, a WS7III and a WS7II. they both have more power than either of my cousins GRP's and they both rev out far beyond any GRP i've ever seen. The WS7II has gone through 10gal before I had a tuning mishap and scored the sleeve
but i've now rebuilt it and its going as hard as ever. I even ran it in on the same old rod. i have now changed the rod but there has to be something said about a motor that can do 10gal and be run in twice on the same rod before it needs replacing.
Novarossi micron match there pistons to there sleeves. This gives your engine a much longer life. the run in is less stressful on the bearings and rods so that the life of the internals are extended.
So in my opinin you cant go wrong with an RB
Another motor a lot of people around here are now running is the new Sirio. it screams and has been wearing quite well. so it might also be worth a look