I have both the 135 F2 and 200 F2 and with the 200 and IS you can easily shoot 1/30 or 1/50 all night long and if Sigma can do this, it would be like a mini 200mm F2 (and probably a lot lighter).
Actually, it'd be a mini 200mm f/2.8.
135 / 1.8 = 75
200 / 2.8 = 71
200 / 2.0 = 100
A mini 200mm f/2 would be a 135 f/1.4. And that would not at all be small, lightweight, cheap, or discreet. Imagine the bastard love child of an 85 L and a 200 f/2.
But this rumored lens, if it becomes real, would still quite impressive nonetheless.
What I meant by 'mini' is the 200mm f2 smaller brother. Going to weddings and shooting a 135mm at 1/50 SS could save you a couple stops of noise. From your example, the 85 / 1.2 = 70 could be the smaller brother to the 200mm f2.8 ?