If it is any help, MB had an article about the 32nd PT boats a year or two ago, and I put R/C into the Lindberg one in the mid-80s, with various configurations. The best had two shafts geared to a 540, with, I think, 35mm twinblade props and a 7 cell1.2Ah nicad pack, and went like a bat out of hell. (See Airfix magazine, 1984?).
Vic Smeed published a Schnellboote plan at 1/32 through MB. Not difficult to scale down, in sections, with a computer/scanner/printer.
The 1/35th scale version would have to be lighter – about 3/4 the weight – so smaller battery – but no doubt the same energy, after 30 years of development.
The Schnellboote should manage pretty well on a 540, and with more displacement available for batteries, as it was almost twice the displacement of the PT boat.
Cheers
Mike