Right now I'm on my 3rd EAA Witness and absolutely love them!
I am interested in the TA-90 ... as it is definitely in the same CA-75 "Clone" Family.
As for the slide stop: From the exterior photo views, it looks like the Witness stop may work. I would call EAA and ask.
Further, here's a good link on the Tanfoglio Witness (although the guy that wrote it no is longer involved with Tanfoglio pistols)
http://savvysurvivor.com/gearrevewta...ies_pistol.htm