Interesting, i wonder what SR forums your reading, as neither of these basic mistakes is a reason why mages are OP.
It has much more to do with the fact that unless i'm a mage, my mage companion has a LOS on me or i have one of the 2 anti magic surge qualities, a half way desand combat mage can knock me out with little effort and there's absolutely nothing i can do about it, except try to kill him first.
OK, am I missing something or is your definition of "half way decent" way different from mine. I know I can be pretty ignorant on magic, so if I'm missing something here, let me know. A non-initiate Mage firing off a stunbolt is probably using Force 5 or 6. Let's say he has 12 dice for an average of 4 hits and a maximum of 6. That's resisted by Body and even an average person is going to knock one off that total. Its going to take more than one of those to knock anyone out. Admittedly, most mages can resist this drain all day long.
If the same character overcasts for Force 12, blows an Edge (based on having 5 Edge), he's going to have around 10 hits to be resisted, 12 maximum. This will eat a non-specialists' Stun track pretty quick, possibly even being a one shot KO. The drain's going to be doing a point or two of physical to him per shot.
Initiation and/or foci can boost that somewhat but at a cost of Karma and nuyen both. Initiation (and matching magic increase) are actually a very Karma intensive way to get another 1/3 hit per level. Up to Force 9 drain resistance on the low end isn't too big of a deal. On the overcasting end, every Force over 12 is another physical hit to worry about.
Take as given that a Mage has four IPs due to spells. In a toe to toe standoff with a Street Sam with all else being equal, the Mage goes second (either they both spend edge or both don't). He can't go on Full Defense because he wants to fire those stunbolts. Sammy fires a short bust of stick'n'shocks for 8S+1 per hit. Unless the Mage has 16 points of armor, he just took a shot with more power/hit than he can fire back, even with overcasting. Sure it costs a few bucks on ammo but it can do a one shot takedown at a higher dice pool at no Karma cost at all. Even if you prefer gel rounds instead of stick'n'shock they work out to be roughly equal to a Knockout bolt (ie armor and base damage roughly cancel).