Its ok mate, I'm from england too, I just live in greece...

Can't be sure about the diff, I must say I never looked at this machine at all. I would recommend getting all the metal drivetrain parts you can, but besides, I don't think a .15 is going to cause you any problems, even the quicker ones. I assume you're going from a .12 to a .15? Get all the metal parts you can, and a good .15 engine. At least if it blows the drivetrain to pieces you have an excuse to get a better machine...and you will already have a decent engine...maybe get an HPI MT kit, they're cheap-ish and great bashers...and they're 4wd (the pirate is 2wd innit?)...

Of course, you could consider selling the pirate on ebay, and getting an MT18ss, it's only available in kit form, and you already have radio gear, obviously, and it comes with a very fast .18 engine (and a two speed, if I remember rightly). Clocked at 72 kph out of the box (nearly 45mph) and it has bags of torque compared with .12s...