I wish I could offer a comparison based on Dana and my records since we switched to the short shank Partridge Nordic Single Spey (Boile Carp Hook). However, since single barbless hooks are the law in British Columbia - and have been for many years - I have no experience with multiple hook points.
I can confirm what Ted reports about our hooking to landing ratio since switching to tube flies. While using standard salmon hooks our average was about 50 percent - and had been for years. Now in the start of our 4th season with tubes and the short shank hooks - it is hovering around 80 percent. The increase may not hold up over a longer sample - but it is hard to argue with the success so far.
Much of this success, I think is the Nordic Single Spey hook itself. It is quite short in the shank, with a very wide gap, as well, it almost looks like a circle hook. In fact I often have a hard time getting my fingers on the hook to remove it from the jaw of a fish - they really hold - and there is little leverage for the hook to work loose.
Maybe my ethic of believing the singles are better and healthier for the fish is born out of legal requirement. However, I do believe that the relatively small single hooks that tubes allow us to use have to be easier on the fish than multiple point hooks and even large traditional singles
Our biggest problem is getting suitable hooks. The Nordic Single Speys have in the past been somewhat difficult to find - as well, the eye of the hook is just a little too big for our liking. I know Dana has tried unsuccessfully to convince the people at Loop to make a single point version of their double - it seems they don't believe there is a market. I think they are missing the boat.
Malcolm, if you continue to experiment with singles, give a short shank hook - particularly the Nordic Single Spey a try.
Tight lines - tyler.
Still Living Large!