It's been a while since I have posted, so that to the OP for this post.
First off I would like to address a few things:
1) High AR vs Low AR argument:
In my experience the High AR (8+) wings are not as easy to use as low AR (5 or less) wings. That being said, there are huge differences between wing shapes and more importantly size so it is difficult to compare the different models. However in general a Low AR wing will need less speed to foil stably. They will however reach a point where they do not go any faster, and this can get boring for some. This is why I designed big medium AR (6.6) Wings for our first production run.
Why are high AR wings generally faster? In theory a high AR will have less induced drag e.g.. more speed. This is talking in very simple terms. There are a hell of a lot of other variables that come into play. eg. Profile, Wingshape, angle of incidence, washout etc etc. The list is long.
High AR wings are typically not as roll stable as low AR wings. But here again, this is an oversimplification. If you compare two Wings that have the exact same profiles, sweep, AoA etc. but two different ARs then the Low AR will always be slower and more pitch and roll stable. But when you design wings you will not just scale on design to a different AR and leave it at that. So the argument is moot.
Stability is also controlled by things like the use of vertical stabilisers, an/dihedral.
Another big factor for roll stability is the span of the back wing.
2) Stiffness of the strut( main mast) is all-important to Torsion is death.
One of the most important factors that a lot of people overlook is the flexibility of the strut. You can have the best wings and fuselage, but if your mast is twisting too much this will ruin everything. If your plane (wings and fuselage) is moving around you will have major problems controlling it as speeds.
3) What is really faster?
All theory aside, the simple truth is: You are only as fast as your ability to control the foil. My fastest speeds in a straight line are all with an AR 7.8 Wing instead of my AR 11 or 9. The potential for speed of the High AR wings is there, but I am not good enough to push them hard and still control them at 60km/h+. The AR 7.8 wing is where my level is now and I can push that wing to the limit. That is the biggest factor for me when it comes to speed. If you are in a crossing or race the most important thing is to have consistent speed and not fall. So you should always use the equipment that you can control the best, over what is the fastest and newest.
4) High AR vs Low AR on the Racecourse.
A real high AR wing will have a much higher lift potential and will go higher to the wind (upwind) then at lower AR Wing. So on an up and down course you would want a high AR wing to get you to the upwind mark fast.
However foil racing is not always on up and down courses and also long crossings are usually on reaches. There you want a slightly lower AR wing with fast profiles that is trimmed for reaching.
5) DO YOUR MATH, but spend time experimenting.
I am not a aeronautical engineer. Everything I learned about designing foils has come though trial and error and reading up on the subject. There are a lot of great calculation tables which can give you a good reference on where to place your wings or what profiles you should use.
6) SIZE is important.
The more surface area your wing has, the more drag you will have. Smaller wings are faster, but this comes with a drawback. You will need more power to get up and foiling stably. A bigger wing (HighAR or Low AR) will always get you up and foiling earlier than a small wing.
If you are a heavy guy, build or buy a bigger wing. If you are lighter, you will want something smaller. We all typically have 3 kites in our quiver to cover the wind range we need. This is no different for Hydrofoil wings. If you really want to get going in the lightest of winds (sub 7 knots) you will want a big wing. and for super fast foiling in strong winds you want something smaller.
7) Pitch Stability:
I have been reading and hearing from a bunch or riders that their foils become very pitch sensitive at high speeds. e.g.. The faster you go the more twitchy the foil gets with respect for height control. I for one cannot confirm this. I find, the faster I go on my foil, the more pitch stable the thing becomes. If your foil actually becomes more pitch unstable at speed, then it's not balanced. The rear wing should be stabilising your pitch and fighting against the pitch moment more the faster you go.
So if you foil is dolphining up and down when you are going fast, you should check your rear wings. There is something wrong with them and they are not doing their job.
Another factor on pitch stability is obviously the profiles used for the front wings. Some profiles are more pitch stable than others.
Take all I say with a pinch of salt.
All I am telling you here is from my experience of developing my own foils for the last 4 years. I am not all-knowing and there I learn new things everyday. That is the fun of working on foils.
I may also be totally wrong about a few things and will retract what I say in a few months, but for now this is how things work for me.