I have no data for the stacks with or without dimples, I just copied the idea and let my customers decide. Almost all liked the dimples so that's how I make them. Maybe the dimples work at lower rpm/air speeds and then the short stack comes into play at higher rpm?
I suspect HRC have done their homework, although maybe for our more standard RC's its more cosmetic?
If you are interested in bell mouth shapes there is an interesting article on the internet by Prof. Blair of QUB, he suggests that an elliptical shape of the stacks is the best. As soon as I will be able to do some machining again (my recovery from surgery on a broken hip is not going to plan...) I plan on trying this. At the moment I use a radius of 25mm, that's the largest I can do on my lathe and mill.
SP1:PVM mag.,full Akra,Durbahn airbox,T1 G2 snorkel,SpeedCell,SP2 swing+mod. subfr.+engine cover,WP susp. +TiN,endur. fairing VFR400 headlight,KYA fronts, Setrab oilcooler,MR clipons,flowed throttlebod.,lightened flywheel, Moriwaki rests, Delrin stacks,BKG triple clamps,PC II,Ti bolts, qr seat+tank,Durbahn/ISR rear brake,Durbahn CCC,AFAM 15-41, DID520,CBR600RR coils,50310 fan switch,mod. wire harness,HRC overflow,Extreme Graphics undertray,Harris fairing stay.