As I said earlier (post #55), in TMOST on page 191 it DOES say that "impulse engines" are STL. I believe that portion of the book came straight from the writers guide. In any case it seems clear by that quote that impulse=STL. There is no documentation that indicates anything different.
That's not the same as dialogue on screen stating same though. There are things that are not accurate about the Enterprise in TMOST or writers guide. There is nothing from the filmed TOS episodes that indicates Impulse = STL.