Hi, Manwe. This is a common question being asked again and again. (I was one of those people doing the same few months ago)
Most of my information came from Jonas, Godfrey and Fred. If you searched their threads in DPR, you would get plenty of them.
I can summarise a few things for you to the best of what I knew so far.
Fa 77 ltd shares similar optical design to A* 85 f1.4 (From DPR Fred's post)
A* 85 f1.4
Fa* 77 Ltd
All 6 elements in 7 groups. Structurally very similar. But optically it shall be quite similar. OK. A* 85 f1.4 should be tad sharper but it would be compromised by its own fringeing problems...
Fa* 85mm f1.4 is then quite different compared to these two lenses shown above
So Fa77 mm is not much different compared to A* 85 f1.4 in structure.
Of course there are two different cults supporting one or the other.
The fa77 ltd is sharp wide open but not that ultimately sharp. But stopping down the aperture makes little difference in sharpness which extends to corners. The A* 85 f1.4 tends to have flare and have dramatic chromatic abberations wide open til stopping down to f4 or so. Then A* 85 f1.4 is reported to be sharper than Canon 85 f1.2L at f1.4 in one review testing which causes an outbreak of group hysteria in DPR canon SLR forum...
I personally want to get A* 85 f1.4, just to see how different it is comparing to fa 77 ltd. There were 3 copies on ebay sold last week: all in bad conditions costing 850 Euro each roughly. 2 of them from Poland actually ...
Now Fa77 ltd is pricing similarly to A* 85 f1.4 prices on ebay. I think whenever you can lay your fingers on any of them, it should do you justice for concert shots (maybe not long reach enough?) And you are going to use it in dark environment, CA should not be a major problem.
Maybe Fa 135 f2.8 will be a better option for a concert or even Fa* 200mm f2.8 will do a better job? Or the forth coming D FA 200mm f2.8...
Da* 50-150 f2.8 might be also a good option (I am sailvating for this lens too)
LBA or GAS never stops hehe
Cheers