here were also passenger ships capable of faster speeds, and we even reference those in some of the unit details of the higher transport ship levels. So while it may not have been that common to use many of them, it was technically possible and not a fantasy.
Comparing a handful of high-speed passenger liners to heavy cargo ships is comparing apples to oranges. And your "very knowledgeable" colleague should know that. A converted passenger liner, such as the RMS Queen Mary, could cross the Atlantic at 25 to 30 knots (46 to 56 kmh), but her ability carry heavy military equipment and other cargo was limited.
If you're going to have a WW2 game, based on WW2-era tech and tactics, let's use actual WW2-era tech.