I posted this thread in GD of normal forums too, and there was some discussion on how much dmg the ult turret does. I did some number crunching for optimal scenarios which I think is interesting:
turret auto attack:
90/110/130 + (.33*ap) per hit
8s duration with 1.05 AS means 8 attacks (assuming targets in range for full duration)
total possible: 720/880/1040 + (2.64*ap)
Beam attack:
225/300/375 + (.8*ap) per target per hit
has half of the 12s base cooldown of the normal turret so 6s cooldown means 2 fires in the 8s duration
total possible (per target hit, pre CDR): 450/600/750 + (1.6*ap)
but the cooldown is affected by cdr, so with max 40% cdr, 6s goes to 3.6s and gives a max of 3 beam shots. (assuming 1 beam fires instantly on placing, the CDR cut off point for still getting 3 shots in the 8s duration is 33.333%, we'll say 34%). So:
total possible (per target hit, with >=34% CDR):
675/900/1125 + (2.4*ap)
So the total max possible single target damage over the 8s duration is:
1395/1780/2165 + (5.04*ap)
which averages in DPS to:
174/222/271 + (.63*ap) dps
Of course this is all just math and how it actually plays out situationally will be different, but even looking at the dps, that's pretty repsectable. Compare to fiddle's max drain at 180 + (.45ap) or malzahar's ult at 100/160/220 + (.52AP).
And that's if the beam only hits one target. if you want to talk about a modest dream scenario where say you are in a team fight and all 3 beam shots hit say 3 people, the damage comes out to:
2745/3580/4415 + (9.84*ap) total multi-target dmg delt
or in dps:
343/447/552 + (1.23*ap) average multi-target DPS delt
Like I said, this is all theoretical. Don't know how much it means, but those be some crazy numbers!