As I'd rather not drill an extra set of holes and bend the masts I'll try the less raked option first once the hardware shops open again here after their long summer break so I can hunt down some 'jack chain'.
Jack chain may not be a good alternative for you then. The fins need to be some distance away from the outhaul so they can cross from side to side. Jack chain, being rigid, might jamb up against the outhaul rather than passing under it. At the very least you would get a lit of clicking and clacking.
OR, you could try mast extensions -- 1/4 inch wooden dowels that slip in the bottom of your masts. 1/4 inch length is about all you can do safely. In this picture, you can see the effect on the front fin:
The last possibility is the "ring clips". I started with these. Problem is, if you are a strong pedaler, you can unravel them and have your fin slide off the mast. Fins don't float! Use them cautiously and with frequent inspection.
This is actually very interesting, I tried opening the adjusting screws further and pedaling does become easier yet I can detect very little loss in speed if any (interesting).
As you know, the dynamics of propulsion have a lot to do with blade pitch, RPM (CPM in our case), boat speed and load. That's the amazing thing about the Mirage drive -- it's effectiveness across all these range factors!
If you loosen the clew outhaul, it does make it easier to pedal and your efficiency through the kayaking speed range improves. As speed increases though, your cadence must increase to maintain the best fin angle-of-attack (AOA) on the water. At your speeds though (8 to 10 MPH) your fins are more likely to slip-stream if the outhaul clew is too loose unless you're cadence is about 90+. So your case is a bit unusual -- you would possibly tighten the outhaul for a better high speed "bite", especially if you're pedaling in the 40 to 50 CPM range..
However when sailing and pedaling at the same time (what I always do) my pedals now oscillate and shake quite a bit while underway with the adjustment screws turned out quite a ways. It's possible that my flippers are just all stretched out (which they appear to be from so many pedaling miles)... my boat averages between 8 and 10 mph average speeds (regardless of wind conditions). I guess my question is with your adjusting screw lengthened the way you have how do you prevent your fins from shaking violently.
If your fins have a warped leech like this (below), they likely are pulling to one side and may be causing your vibrations at those speeds. They could also be causing one leg to be more fatigued than the other , having to work against the fin bias. The current Turbos don't stretch like that and should perform much better.
Also, if your Drive is that worn, your parts may well be causing vibrations, even though it functions perfectly well. Mine doesn't act that way, even at those speeds, so I'm not sure what specifically might be causing that effect. I have a bad habit of breaking parts that get worn, so I tend to preventively replace when something starts to show signs of wear or fatigue.