Here's my 10 cents' worth.
I am a fat old bastard with a crook back, so moving around on deck was never an option for me. So I sit on my skipper seat up front on my TI, and have a ball! I just spent 2 days on Port Stephens, pedalling for hours on day one, but screaming around today with half the sail reefed. My Lowrance plotter showed a peak speed to windward of 7.8 knots, not too shabby, although running into waves sent buckets of water at my face and over my head.
I can understand your enthusiasm to sit out to windward, but my experience is that you get so much fun for mile wherever you sit, to a large extent you can forget the typical off the beach sailing scenario.
As for stability, I have tested, and a TI ama needs 150kg downwards force to push it til the top is level with the water. That is a LOT of resistance to heeling!
The biggest secret to going fast iwth a TI (and probably an AI as well) is to furl the sail rather than stagger on thinking you are going fast when you are not. Today, reefing the sail halfway, with no other changes at all, increased my average speed by a whole knot, as the hull sliced through the waves rather than smacking them. Hard to explain, but my plotter doesn't lie.
You can control mainsheet trim and steering from either seat. Frankly, (and this is based on a fair amount of experience sailing quarter tonners and J24s) I generally sail with the mainsheet cleated, not touching it through tacks, and concentrating my efforts to passing through waves with minimal fuss and loss of pointing angle, and usually only playing with it when off the wind. As Keith mentioned, Hobie now make a hiking stick for Islands so you can steer from outside the seats
You will also often find that it is worth pedalling the Miragedrive in at least the following sailing situations:
Tacking in really light conditions, to enable you to smoothly glide through to the other tack
Pedalling like buggery when tacking in strong winds, to avoid being pinned in irons and discovering that you rudder is now working in the opposite sense when you start going backwards!
If you are having trouble clearing an object to windward and do not wish to tack (or your path on the other tack is blocked), you can pedal vigorously and help the sail to point higher.
While some sailors take the Miragedrive out and fit the blocking piece for less drag, the above scenarios tend to make things busier if you try to also use these techniques, while sitting out.
In summary, it is easy to see shortcomings in the performance of the Islands when compared to other off the beach sailboats, but versatility is the name of the game for these unusual craft, probable best demonstrated by three of us last night. In a sheltered bay in Port Stephens, Stringy set up his tent on four padded hakas beneath his "powerboat" bimini on his AI, Slaughter slept in his hammock suspended over his AI, while I put up a tent on shore. Stringy brewed up numerous coffees for us, and we each had small butane stoves for food cooking and eskies for keeping stuff cold. Try all that on a Hobie 16!
2012 Tandem Island "SIC EM" with Hobie spinnaker!