IS is a far superior system to post mount. Post mount is easier for duff mechanics to set up but most importantly is better for production engineering. Much easier to assemble bikes on the production line with post mount.
Its a dreadful bodge using slotted mounts on a brake and is heavier and more flexible
Not so sure about this. If the brakes were on a car the bolts wouldn't even be done up tight and the calliper would float/self centre themselves. Not sure why it's heavier either, but can't see why it would be inherently lighter either?
With PM when you brake, you load the mounts along the line of bolts and the push/pull is against the thread, with the force being transmitted directly into the fork leg in it's strongest plane. With IS you have a shear force across the bolts and this load is transferred sideways as well, so the mount itself is under greater stress.
The above is a marginal benefit, but PM is easier to setup and adjust for pad wear, or a bent rotor and it's also much easier to change rotor sizes too. Only disadvantage vs IS that I can see is that if you wreck one of the threads, it's the fork lower you've ruined not a half caliper.
Anyone remember how you hcanged otor sizes with Hope brakes? you bought a new half caliper. PM is progress.