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.
I think what was implied by that line was that transwarp drive didn't make the Excelcior better
than the Enterprise. I think this is supported by the other line about that ship at the end of IV.