To clarify the terms.
Flex is the "normal" twisted wire thing that many folk use.

Wire drive is the solid piano/music wire. 0.062 or 1/16" is a common size. Few use smaller and big power needs bigger.

I run 062 wires in 6 hulls.

The best I have is a 7XL on 4S lipo to a 642 prop giving 55mph +
That is over 1600W of power.

There are far higher power applications that I have read about but not yet tried.........on the water at least.

Jeff Wholt carries biggerwire drives as well (.078, and .093) on his site (www.rcraceboat.com), thanks for the reply Simon!