I need to get some feedback on a mod that my friend did to his rxp.At first he was running 70.8 @ 7950 rpms with a stage 1.We added a Riva/Vortech impeller and made the waterbox shorter and stripped on the inside and did the thru-hull exhaust.Now he is running slower and the rpms dropped to 7800.Would this be due to having to much exhaust because of the waterbox mod?Or is there is something else that caused this problem.He has a factory 14/19 Solas Concord.