When it came to engineering, Scotty was always a straight shooter. He never would have implied that the Excelsior had transwarp drive only to the degree that his grandmother with wheels would have been a wagon, if it weren't his professional opinion that the Excelsior's transwarp drive couldn't work.
There is personal pride involved where the Enterprise
is concerned and something like the Excelsior
would eventually push older starships out of service.
And why would Scott need to pull anything out of the Excelsior transwarp computer if he was convinced the technology wouldn't work?
Star Trek III wrote:
SCOTT: Aye sir. The more they over-think the plumbing, the easier it is to stop up the drain. ...Here, Doctor, souvenirs from one surgeon to another. I took it out of her main transwarp computer drive.