Thanks for the replies!
I had to go and double check myself just now, but STI 10mm mags fit in the .40FS frame (upper/etc. is in pre-op across the country), strangely one fits nicer than the other and both are NIW? I struggled to find the two mags I have, and had to order them from two different suppliers as I was scrambling to get these right before CO mandated that 15rd mags are all I can purchase here. I know nothing about STI's, much less if there are mag dimension differences based on production/etc./etc.?
I double checked the Para 16-40 mag also, and it is just a bit looser fit in the .40 USP frame.
I looked long and hard for a conversion barrel to fit my Expert 45, only thing I could find was a .400 Corbon barrel that was all cobbled up with a DIY magnaport job, and lots of "no's" from the usual sources of barrels for these things.
Oh well, so it goes.