Late last year, one of our members here provided me with a set of 13" standard Progressive 440s for the SERG after they converted to Bitubos. Just now getting warm enough in the garage to spend the time out there to put them on, so that's what I did yesterday afternoon. Incredibly, these shocks set the bike about an inch lower in the rear than the stock air shocks, even though they are both claimed to be 13". For comparison, I lined them up next to the 12" heavy duty 440s on my SEEG and they are definintely taller, though closer to only 3/4" taller instead of a full inch. The lower height on the SERG is a blessing since with the stock shocks, the bike had a rake to it that set the rear end of the bike a little too far up in the air. The Progressives pretty much level everything out.
The bike now has the familiar (and much improved) ride quality of the SEEG and though I'll still need to keep air in the front forks, at least the rear end is good to go.
These things look so much better than the black air shocks, it's almost a shame to put the bags back on.