Hi Fred,
My one of my fishing partners put some on his mid-'80s pro-line 21 wac (tank of a hull for a 20-21) and although I have limited run time on it, it does appear to reduce his mpg even though he planes quicker. I'm not sold on them but the price and simplicity are certainly right. I'm not sold on their ability to do what you'd like to as far as managing your trim in a mixed-up sea or flat out snotty conditions... great question though. Be interesting to hear from the engineering wizards... look out! here comes team Bennett
Gillie