I'm guessing you have the long shaft rather than the XL if you have a 13 7/8 19p viper Subaqua?
I can achieve the same top speed as you on a good day since I lifted my engine to the top hole but I have a considerably smaller boat which is countered by the big V6 gearbox and the 2.25 gear ratio on the XL.
At the same engine revs my prop is spinning about 12.5% slower than yours so you must have allot more slip than I do.
Just out of interest what WOT RPM can you pull with 7 divers and kit onboard with the 19p viper? I'd be looking to a 4 blader for pushing those sorts of loads, probably a 15p.
Which props from Apollo did you go for Gordy? Where did you source them? The Enertia is looking like being about £600 with the flo-torq kit.