too much helical will slow the arrow down a lot faster...
Correct
...and when the arrow slows down so does the RPM's which will reduce broadhead stability.
Incorrect. Anybody that's ever shot and hunted with a stickbow knows that's... uh.... an uneducated statement. Who knows better about broadhead stability on slow arrows? Overdone helical will slow the arrow faster but the parachute effect keeps the broadhead perfectly stable and flying straight.
eightwgt, that's nowhere near 'too much.' Looks like you've got enough space to shoot through a prong rest and get good clearance. I've shot feathers that were helical'ed so much that the leading tip of one feather was wrapped past the trailing end of the next feather (off the shelf on recurves and longbows). Arrow flight was excellent and I never found an arrow fletched that way that would let any well-mounted broadhead - even big, unvented 2-blade heads - plane off course. A bit slower, for sure.