I think the way to be effective as a sniper-mage would be to
1) Make sure you are taking advantage of the one thing guns can do that spells can't - shoot through walls.
2) Use spells to back up shooting and/or do different things. The cardinal rule of hybrids is "don't make a dude who sucks at throwing lightning bolts and sucks at shooting guns."
So based on that, I would be using Heavy Weapons (with a Gauss Cannon) for maximum shoot-through-walls-age, or if you dislike being good at things you can use Longarms. You could either be a full mage or a Mystic Adept (depending on whether you are using SR4+FAQ [no] or SR4A [yes]), probably pick up Spider and use Illusions (Physical Mask, Trid Phantasm, and Improved Invisibility being the better ones) to get into place, and Detections (Astral Window, Mana Window, Clairaudience, Extended Detect Life, and Spatial Sense in particular would be good) to find people. Don't bother with touch spells (they do nothing but add multiple points of failure) or unarmed; you should probably just use guns close-in too, and just know Stunbolt for a concealable "weapon."