i did exactly a similar thing except long pipe and white tube then was told by a blunt yorkshireman that trying to make it into a 250 was daft, and to start from that.
black tubes are no help if you need a big handful and if you dont why a 300?
its not an upgrade at all. If you are finding the pwer too much chop it in and get a 250.
the disadvantage with a weight is that it allows the motor to run on which for a less than expert rider in such a pwerful motor can cause more trouble than you think it does. Exception is possibly in muddy southern trials it can help with traction.
A 4rt is a bit of a marmite bike. Certainly not heavy till you get in trouble then feels like its fitting you, takes a while to master them in mud too, imho.
That is the reason.
I like andy have been to a few in the past and im struggling to think of a WTC that WASN'T supported by some local or regional authority.
The round in Fort William was there specifically because of the tourist board support?
In the uk speak to steve saunders he will advise.
Whether its a bad batch of oil based on two completely different makes of bikes is an assumption too far imho.