I've no personal experience with that engine, so I can't tell you whether or not it is the most powerful there is, however there exist some very powerful nitro engines on the market, I would suggest that you ask other GST owners what engines they have used when the stock one eventually wears out, I expect some of the racing engines would be a bit more powerful, but then they should be, they cost a hell of a lot more.
Engine sizes for cars generally range from .08 to .10 micro blocks, .12 to .18 (with some exceptions) small blocks, and .21 plus big blocks. This is expressed in cubic inches. When you see the higher figures, like 4.6, they refer to cubic centimetres. 4.6cc for example is .28cu, and you'll more often hear them referred to as .28s, rather than 4.6s. You can type in google.com 'convert 4.6cc to cubic inches' and it will tell you. So a 4.6 is a .28, etc.
The GST does have a huge engine, so it's bound to be very powerful on the bottom end to pull those wheelies in such a big truck, try it and see.
