Interesting thoughts. How would one go about efficiently using tactical scan for maximum murdering capabilities?
It depends heavily on the weapon you're using and what you're attacking, but the worst thing about Tacscan is the somewhat lengthy (compared to other debuffs) animation, so its basically about trying to work that in as seamlessly as you can. My favourite way is using charge weapons (Arc Pistol, Kishock etc.), where you can fire> begin charging> use Tacscan while still charging the weapon. You can also cast it through walls (provided the red box comes up) and around corners/ over cover without exposing yourself, so if you take a bad hit just use it on the nearest boss while recovering and it'll last quite awhile. If you can't use any of these, I'd reccomend simply casting it before anything else to make full use of it on all your attacks. The miniscule cooldown lets you use other powers almost immediately anyway.
I max it on all 3 of my sexy manquarians because a 32.5% multiplicative debuff is the biggest single one in the game for glorious quarian bragging rights. Secondarily, it helps everyone in the server (especially pugs, who often bring low DPS) so you can blame them for lack of teamwork if you use it a lot and still wipe.
You can also do an interesting and very survivable (if not really min/max) build using the 6b Area Scan as a sort of ghetto geth scanner. Equip Drill rounds, scan something in a spawn (preferably a boss or sub boss) and light everything up, then spam grenades and bullets through walls at things. It's especially effective with the QMI and a good sniper rifle (Javelin, Widow, BW, Valiant etc). Fun, and lets you keep grenade gear while still getting wallhax.