A couple things, odds are your average full up mage in SR is gonna around with a small number of spells up with high regularity.
Your drain stat increases, your initiative increases, your enemy detection spells, maybe combat sense or an armor spell. Odds are most SR mages will have something from that list active, most of the time starting character will run those on low force sustaining amulets, but there are other fairly easy ways to do this (personally sustaining the spell, and having focused concentration, or spirits of man). So just things to consider when you start having more then one mage in the room.
One mage with single spell quickened is probably not going to be a huge threat, yeah the mage would do well to avoid the magic police, but most folks aren't going to have clue. Most of the spells I listed aren't going to have visible tells, and most places don't have any means to check if your walking around with magic up. So odds are your never gonna run into a bouncer who's gonna ask you to leave the quickened spell outside metaphorically speaking of course.
Further if your about to throw down as spellslinger vs other Spellslingers. The mage with one quickened spell probably isn't your top concern assuming there are more options. The guy/girl you worry about is the one with a bunch of spells active be they quickened or just otherwise sustained. And the target you worry about more then that girl/guy, is the one who's aura doesn't match what your seeing. Obvious fake aura, means the target thinks he/she is stronger then you and doesn't care if you know it. That's the guy you treat like he's packing a panzer cannon.