Theo and Charlie ..., my understanding is that the SE upgrade is not just superficial, although I've heard that the round buttons increase horsepower by 15 percent. LOL :)
Ok ... jokes aside, the SE upgrade incorporates some of the technology changes in the Ref Anniversary 40. As Theo mentioned, increased power supply, Teflon coupling caps, and other sundry circuit changes.
I don't recall reading how the LS 17 SE compares to the Ref 5 (non-SE) version. For some reason, I seem to recall that the LS 27 nipped at the heels of the Ref 5. That in part may be why the Ref 5 was also "modded" to an SE version. I owned the Ref 3 and Ref 5(which was upgraded to the SE version last February).
While I thought there was some refinements between the Ref 3 and Ref 5, I questioned whether my investment was worth the money. IMO, I think the SE version is a marked improvement over the Refs 3 and 5.
In short, I surmise the LS 17 SE version is worth getting, round buttons aside.