Check here, here and here (performance isn't affected at all on screen mirroring as it can be seen on Q3A photo taken).

I have ordered 2 TGS sticks and will order soon 2 DB9<->USB adapters from Retronic (I have 2 already and are confirmed to work flawlessly). Hope to conclude some tests on 4-way e-uae setup within July.

Think this is pretty much the best possible setup for multiplayer on classic amiga games (apart from a video wall/projector).