I haven't but I went with a 61 RE since the ARC's are a touch on the heavy side. I think they build to 7.5 lbs dry which IMO is at the upper end of the capabilities for a 55AX in a pattern plane. A scratch built Calypso on the other hand could probably come in at under 7 lbs, maybe 6.5. At 6.5 and even 7 lbs, I would definitely consider the 55AX. I find the model looks so much nicer with an inverted RE 61 though. I'm sure the only reason Hanno didn't run the ST 61 TST as he did in the Magic was because they wanted him to promote their new S61 SE engine.
David