One of the main problems with high-force spells is that they're obvious. The threshold to Percieve a force 5 or higher spell is 1... not necessarily a good thing when you're trying to quietly stunbolt a guard, as even the dumbest security system AI can spot it on a camera, and if that guard has any friends around they're going to get the idea that something's up. Suppressed gunfire or a hand-to-hand takedown, on the other hand, can be a lot quieter.
Once it turns into a serious balls-out firefight, your mage is *not* going to have time to clean up a half-dozen high-force spells, unless your runs tend to happen in the middle of the Alaskan wilderness. Having the mage be unconcious at bug-out time makes cleanup hard, as well. With security response times measured in combat turns, not to mention the fact that any time spent cleaning is not time spent heading towards the door, having your mage unload huge amounts of mana during a battle is a quick way to have someone pounding on his door shortly thereafter. There's ways around this, of course, but they're all time-, nuyen-, and karma-intensive.
Sure, mages can be absolutely devastating to a theoretical security team operating in a vaccum. If you're not following that up with the responses (such as astral tracking) that the SR world has dreamed up, those mages will keep being as devastating as the troll who carries his HMG to the mall, since the cops never detain him. I can strip a submachinegun and throw the parts into a metal grinder, and as long as I didn't leave much in the way of fingerprints, blood, or other evidence at the scene of a crime, I should be good to go. A mage smears his signature all over the place when he uses his talent, and that is one of the serious limiters on their power.
