I used the vice clamp method on my FCU as well with success. I took virtually all of the vertical play in slide to frame fit out, I only left a little bit of lateral play for reliable function. Had to file down / refit the FCU rails to slide rails afterwards, otherwise the slide would bind terribly on the FCU rails. However, it had a rather cosmetic effect, I get pretty much the same groups as before, with slow fire at 25m I can do about 3" if I do my part, more or less the same as before. My barrel to slide fit was good from the get-go, no discernible play when I push down on the barrel hood. Honestly the biggest effect on the size of my groups was achieved by a trigger job - I managed to get down to 2.5lb with about 3.5mm reset and buttery smooth pull, pretty goodtrigge if I say so myself. But for some reason, I am still less accurate with my legion at distance of 25m or more than with a Glocks, XDMs or 1911/2011s for some reason. All in all, I wish the legion had a bit more accurate barrel with a tighter rifling (I found out it shoots best with .358" bullets, my other 9mm pistols are good with .357"), but for the intended purpose (IPSC shooting), it is accurate enough.
