You need to read a couple of the sticky threads here that describe how to load safely, how the hot loads are approached, and that no one else nor this website are libel for you trying something that you're not skilled or following the directions described. For some reason the tone of your post gives me the impression you want to go "hot" without taking all of the time and effort of doing careful work-ups. I could be wrong, so my apologies if I'm busting your chops inappropriately here, but if I'm right, it's better that than blowing up your gun and picking shrapnel out of your hand/arms/face...
The short answer to your question is - the Glock barrel has looser dimensions than the custom barrels, and therefore, max loads will be reached quicker in the load development cycle (and therefore at a lower velocity level) than with a custom barrel. Going too far in the factory barrel leads to the Glock Smilies you allude to.
If you push your brass to that point without slow, careful load work-up, I agree that you should trash that brass. If you work up slowly, you'll see the bulge starting before it gets horribly bad. That's the point you should stop at. (Actually, if you're using the techniques taught here, you'll never get to that point...) I still relegate that brass to the pile that I shoot lighter loads with, but it's not unsafe to reload.
I believe it's Redding, but it could have been another company, just put out a die designed to iron out the Glock Smilie on 40S&W brass. (And, of course, it works on 10mm as well.) Personally, I think this is meant only for the slightly swollen brass, and NOT the ones that have full-blown smilies with a pinched area where the smilie meets the case wall.