Most tracks will let you run a RTR ST with the .15 that came with it because its not an advantage at all.
Unless the .15 came with your truck, you will not be allowed to race it.

What .15 engine are we talking about anyhow? I only ask because most .15 engines are sold with RTR trucks AND there is not an RTR .15 that can run with a .12 that is designed for off raod racing.

I think the .12 engine rule is a good one. It doesnt limit costs as much as it levels the playing field. They have to draw the line somewhere. And no, faster is not always better. Using Nascar and F1 as an example, rules are made to keep speeds down to a reasonable level.