RE: Clew Position
A very interesting question......and one that the answer to depends alot on who you talk to.
One "school of thought" is that by using the upper clew grommet, you actually pull down more forcefully on the leech of the sail and kinda "tighten the leech" a bit.
Basic physics tells me if the boom is the same length, and the clew grommets are exactly the same distance from the front of the boom, you get no additional leverage by using the upper grommet. The angle of the boom running from the mast to the clew grommet will change, but that does not give you any additional "mechanical advantage" (basic levers here).
This could be one of those "feel" things and by having the end of the boom slightly higher, sailors tend to apply more "down force" on the boom with their back hand, and this could conceivably straighten the leech slightly.
Some sailors use both grommets with an adj. Outhaul setup and the boom end gets positioned right between the 2 grommets.
My instincts and experiences here are that it really makes no difference (beyond a different "feel" perhaps), as there is no "physics" to suggest there's a difference unless the grommets are closer or futher from the mast and front of the boom), so if you are taller, use the upper grommet so you don't have a big angle down from the boom head to the clew.
If you are short, like me, use the lower grommet to avoid having the boom higher at the clew than it is at the mast.
But all of this may vary, and you need to go out and see what the effects are with your rigs.
Sail with the outhaul through which ever grommet you usually use, then, without adjusting the length of the boom, simply pivot the end of the boom to the other grommet and outhaul to the exact same distance.
See if you can notice or "feel" any difference.
That's the only true "test" you can make. How does it affect your sailing and more specifically your comfort while sailing. Comfortable is fast and fun. Anything else is not. That's the bottom lihe here.
Hope this helps,