I like the 5506 (candy cane) pipe for momentum oval tracks where you're in a narrow powerband and up near the rev limiter much of the race.
The 5507 seems to work better (at least with our tune) for a track demanding a wider power band and needing more power at lower rpm.
5507 for the long slide classes as well.
I should have checked the old threads (sorry). Some of the new lefty sprint karts run the 5506 candy cane pipe. Is the support the same part as with 5507 (just rotated?).