I have a very superb example of an early 70 Series Gold Cup with the lightened slide along with a matching Colt .22 Cabliber Conversion Kit that was purchased at the same time as the .45. I have replaced (but did not discard) the orignal barrel bushing with a new one that was fit to the slide properly and then mated to the barrel with a final lapping using 1200 grit garnet lapping compound. The fit is as tight as you can get it on the conversion and still have the unit function properly. There is no detectable movment in the barrel/bushing assembly when the slide is closed and the conversion unit functions perfectly using Eley Practice 100, CCI Standard Velocity and with Aguila Standard Velocity. While fitting up the front bushing did help the overall accuracy of this particular conversion it still will not shoot as tight or as consitent as my Model 41's or either of my Marvel Conversion units.
As Bullseye indicated...yes, you can definitely fit that bushing as tight as any match pistol, as long as you leave enough clearance for it to function.
There are a few other things you can do to the Ace to make it more accurate, but as Bullseye said, it'll never be as accurate as a 41 or a Hi Standard..or at least I have never heard of or seen one that was that accurate! Definitely fun guns to shoot though.
I have to back up a bit here. I do know of ONE Ace that would shoot as well as a 41...and it was one of the very early ones with the leather buffer and without that floating chamber.