What is the difference between tight and loose leaches (trailing edges) on wings? Does tightening the leach give you more power and less drag or just choke the wing? Also how does it affect the handling when wing is being dragged along behind you? Just thinking about a nip and tuck. Thanks

At its most basic:Tight = more power upwind and higher angle. This sail will perform better between 30-75 degrees.
Loose = more power reaching and downwind. This sail will perform better 90-160 degrees. The crossover will see the looser sail bring you downwind and the tighter sail underperform.
What does this mean for your specific circumstances- you will need to address what conditions you're in most of the time, and approach this when considering a take up or ease out.That being said, if you're just touching the leach, you won't make a major difference, if anything mess the leach curve and now have a flappy sail. If you want to change sail shape, you're better off doing 2/3 of a seam.
food for thought anyway!

I once measured upwind tracks with an Airush/Starboard V1 (super loose leech,deep draft) vs a Takoon V1(tight leech,flatter).
No difference at all.

The Takoon tacks way better, it cuts cleanly while the Starboard will try to rip your arm off as you change hands.
The Starboard pumps with a lot more power and less technique.

Another thing to consider is what the "wind sees". For example, looking from the front, if your wing has lots of dihedral in the middle and curves down at the ends then a tight leech will not match the leading edge.

What Jethrow says. You can,t look at an isolated part but has to take the whole profile in consideration.
And I,m not an native english speaker so just questioning: but isn't it leech instead of leash? with a leash I,m always thinking about a leg rope.

Thanks lads. Great stuff. I won't rush into it now.