The EB Mods pipe should work great on the JX-21. My guess is that Savoya uses the FLS pipe on the JX-21 due to the increased run time and very linear power band that the pipe has. It makes less Hp than an OS 2050, OS 2060, Ninja JX 2042 (same as OS 2060) or the EB Mods pipe thus why the longer run times.
When I tested the Ninja pipe on my old GRP .21 I was able to get an additional 30 seconds of run time than with a 9853, MSR1005, etc that made more power.