The TTI universal extension is working very well with both the Beretta magwell and the Springer magwell. This is with the Mac-Gar 18 round tube and does fit the EGW gage. Note: Beretta factory 18 round tubes with TTI extensions or Springer extensions will NOT fit the EGW gage.
The TTI retention system is much better than the Springer. I have/had issues with the little set screw on the Springer extensions disappearing at the worst time. I have many Springer products and all have been very nice quality except this set screw idea.
MBX is making a follower / spring kit that will get "24 rounds" in this combination according to website, but I have not tried. MBX has "coming soon" a 140mm magazine for the Beretta 92. I am considering trying the follower / spring kit, but highly anticipating the whole MBX magazine. With the mono tube, I would think about any magwell would work. I have several MBX mags for other platforms and they have been exceptional.