I don't have a real Conn 120S, just a 7b and a FAXX 120S copy. They're probably not as sharp as the sharpest ever Hellebergs, but the rim is more than average flat and with a sharper edge. That's what Helleberg rim means, though it apparently was less so before the 7b and 120S models came along.
On this point, I think the first advice you got here was right -- there's no guarantee at all, that features you like on a trumpet mouthpiece will serve you the same on a tuba mouthpiece.
For your amusement, a cross section overlay from James New, who copied these mouthpieces. All three are from Conn; HELLEBERG is presumably an old one from before the Conn Precision "Conn 2" era, GEIB is a non-Helleberg model, and CONN 1 is from the Precision series but with a much more rounded rim than the Conn 2 (and a very different cup profile - it's a great mouthpiece for contrabass tuba, but unobtainable.)
