Dave...while I have not heard the LS17SE or the LS27 (useful input from me right?

), I have heard the LS26 and Ref 3 (both of which I owned) and the Ref 5SE and I can tell you in general, there is a noticeable improvement going up the ARC line. This does not mean that the law of diminishing returns does not hold with ARC products, it does. But it does so less so, meaning the step up in performance is generally more easily noticeable in ARC than with some other audio brands I have heard. Having said all that, no one can answer for you your specific question, because only you can answer how much a step up in performance is worth to you in terms of your willingness to pay a specific premium. For one person, the $3K price premium is a no-brainer vs. for another who cannot see $3K worth of improvement. That is the hardest question to answer in the high-end audio business because what each individual is willing to pay for that incremental step up is determined by each individual's passion for the hobby, their ability to discern differences in sonics between gear, their finances, and their personality (always wanting to have the best vs. being happy with good value products), etc...
Having said all of that, my advice is the tried and true, which is try to audition both units side-by-side and see if the difference i worth $3K to you. And I suspect your answer will be the very often cited, "I don't have a local dealer who carries both or either product, which is why i am asking you guys."
Hope this helps somehow