I used figures from a hobby turboprop which gives mechanical power output in kilowatts (or horsepower, if you prefer) and it’s basically independent of velocity.

Well yes, that's a turboprop. But fossil fuel wingsuits don't use turboprops, they use turbojets, whose efficiency very much depends on speed, which in practice means that they are most efficient right before their peak mach, up to around mach 2.