I had very good regen on my 2012 XU, When I got my 2013 FX, I felt that the regen was not as aggressive as my 2012 XU. When I got my 2016 SR, It felt like max regen was about the same as in Sport mode on the FX.
I was told that it was because of the Weight. 2012 XU was 221 lbs. 2013 FX 233 lbs. with one brick, 275 with both. 2016 SR 414 lbs. I know that weight makes a difference, but, it feels like more than that. The differencce
between my FX and SR is similar to a passenger on the FX. If I add a passenger to one of my ICE bikes, I know that it will not stop as quickly but, I don't necessarily notice the difference unless in a hard braking situation.
If the weight difference was in fact the only issue, I would notice an even greater difference if I have a passenger on my SR. Using regen only, I do not really notice a difference between solo and passenger on SR.
As far as Power in ECO on FXS vs SR I am guessing that the same weight argument would be presented. My 2013 has only ECO and Sport, so "Custom" and sport. I have my torque turned to 0 on the FX and it still feels like
it has more pep than my SR does in ECO. It would appear that some of the responders on here are using ECO as intended in lower power situations. If you want your Sport mode adjustable, that is what Custom mode would be.