Very heavy clothing ie. Leather or heavy canvas can be a problem. I do not think it would take "significantly more firepower" to stop them. Any decent defensive handgun should perform OK. 9mm being the minimum. If I lived in the great white north I would not carry anything less in the winter.
I read somewhere that thick leather jackets could stop a Glazer. Other than that, a HP could plug up, in which case it would "fail" and act like an FMJ round. If you shoot till stop, I do not see a problem.
My defense mags are loaded 195 gr. Hydrashock, 230 gr. Hydrashock, 230 gr. FMJ, then they alternate between the 230 gr. Hydras and 230 gr. ball. I keep 2 mags loaded that way. If I can't clear my house using those 14 rounds, I've pissed off the wrong people.
Personally, I think you're on the right track....Not only clothes thickness to worry about, it's been reported many perps now wearing protective vests when they are up to no good.
My calibers go up in the winter for this reason with more opportunities for concealment..
__________________ "Is there no one on this planet that can challenge me"? General Zod
This is true, but I haven't changed my ammunition it's been 45 GAP year-round. If they are wearing some protection they'll have to stop at least ten rounds from getting through which I'm confident at least a couple will.