RE: Brison 5.8 best performance
Wow, something is wrong! My 3.2 will spin a zinger 22x10 around 7100. A 22 x 8 3W prop at 7300. The 4.2 should spin a 24X10 at 6500+. The 5.8 should spin a 26 x 10 at 6500+.
Yes try a 3 blade (it will not help the vibration), check your timing & adjust the needles.