AT least with a Romic, results are pretty worthy.
I'm bottoming mine quite easily and having a little bob when pedaling erratically. I'd have to try the following firmer spring. I'm on a 350# now, I may try a 400# in the future. I have a 450# that I'll put on, just to see how it goes but I figure that it will be too heavy a rate.
But so far, I'm not missing the Roco Air, which is to say a lot. The rear end is incredibly plush, traction to spare. The mid stroke feels supple but not overly soft... while the air shock always was ramping up too much and mid stroke was not as nice.
If you're lucky to have the 5th Element 5" rockers for a 2003/2004 you'll can even use a 2.25" shock to get 140mm travel.
Just watch out your FD for clearance at bottom out. Move it upwards a couple mm's. If you go for the 2" stroke shock, you'll not have this issue at all but you'll be at 5" flat of rear travel.
The 2.25" at bottom out....
This pic is not that accurate... I think my FD made contact with the swingarm. However, upon inspection of the FD today, I'm not that sure. Maybe I just rotated it with my foot as it doesn't look bent.