OK everyone, simply forget PCB isolation routing with the 40W!
I got my 40W brute today, and after cutting a bit of wood for the fun of it (what a BEAST!!!), I tried PCB isolation routing like with the 2W. As expected, the focal point is much larger, so less passes due to that - good. It immediately cut the copper, sparks flying! But… after all, it does not cut the copper! What really happens is: It heats up the PCB, something under the copper evaporates (either some glue or parts of the carrier board itself), and if enough pressure has built up, the copper “explodes”/pops. You do not get good cuts. It tried different speeds, powers etc., and by happenstance hit the “sweet spot” where the evaporation happens, but the copper holds. Result: a copper bubble 
See it here - red frames show different experiments (variation in speed, power and # of passes). The green arrow points at the copper bubble. Upper PCB: phenolic paper. Lower: Epoxy.
Huh - I’m just wondering if the 40W has any worth to me now… The use cases I have for such a beast are close to nil… Perhaps I sell it again… But the air assist pump may come in handy… Always these decisions sigh 