Mate, you ask a lot of questions in a lot of threads. Is this the sixth you've started in the last day?
Why not just ask all your questions in one thread?
To answer your question, too much or too little downhaul can affect your sailing. If you don't have enough your sail will be top heavy and unstable. If you have too much your sail will be too flat and gutless plus your sailing may be affected by you busting your back and breaking your mast in the process
As always follow the manufacturers recommended guidelines.
Hey Jezdog, isn't there anyone else sailing where you are? Don't be afraid to ask for help if you see someone around. Most sailors are happy to help out. When I was learning I was always hassling other riders for info and sometimes still do. For something like setting a sail, it is very difficult to say over a forum what is ideal. You really have to see the sail rigged to see how it sits on the mast and the conditions you are using it in.
By the way, I'm not trying to put you off asking on the forum, the more input from sailors the better, it is just that if you are learning, an experienced teacher could help out heaps.
It's a power vs speed and control thing.
More downhaul means less power at the top of the sail. The sail will feel lighter when you throw it around, gybe, flip the rig etc. because the top will go floppy and have less drag. Now the power is lower down in the sail making it easier to sail. The top will twist off easier giving you less drag - it's an apparent wind thing, too much for this paragraph. Basically faster and easier, if there is enough wind.
Less downhaul means the opposite of the above.
Use less downhaul if you need more power to get going in light winds, more downhaul for stability and speed in strong winds.
If in doubt rig in the middle.
Generally speaking the second panel down should be floppy to the middle for medium setting. About 1/3 each way for maximum minimum settings.
Also more for downwind, less for upwind.
...does that make more sense or less???
If in doubt consult Guy Cribb:
www.guycribb.com/windsurfing_technique_holiday0057v01.aspx?awvariantid=1
Excellent question - glad someone asked it. I have a similar, but different angle on the same question.
I find that it takes heaps of downhaul just get all my battens to at least sit beside the mast. Darn near the top half of the sail is floppy just to get the next batten above my boom to sit on the leeward side of my mast. Of course, with that much downhaul the sail is no good at all in lightish breezes. If I give it enough downhaul to be a nice shape for said lightish breezes, my battens protude a couple of inches in front of my mast and then it's difficult to get them to flip over when I change tack. It also doen't strike me as a particularly suitable aerofoil shape for lightish winds.
I'm using a 6.9sm Expression on 4.6m X6 mast. It's the same on a 4.6m WavePro Matrix 2000 mast I have the same problem, but to a slightly lesser extent on my 5.0sm Sailworks sail on a 4.0m PowerX 60%C mast.
I've tried all sorts of combinations/permutations of downhaul vs outhaul, but alas and alack 
Help would be graciously accepted 
Doubtful it's the wrong mast, as NP sail + NP mast should be ok? Are they roughly the same vintage? Also ask your retailer, they should be able to help you get it rigged right especially if they sold it to you.
I basically ignore the batten/mast relationship, and just look at how loose the leech is. I think with NP sails having the top panel loose about 50% of the way in from the leech to the mast would be a good place to start. I think it's okay if your battens sit next to the boom, especially the lower ones, as once the wind fills the sail, they will pull away from the mast and make that nice aerofoil shape that we all love. Take the time and actually look at your sail when you are sailing, who cares what it looks like when it's rigged on the grass, it's gotta look right when you are sailing! And don't forget batten tension, as this is often overlooked and can easily make good sails feel rubbish!
Also, with downhaul settings, you probably only got a 10mm sweet spot range to play with, outside of that your sail will probably handle like rubbish, so once you find that sweet spot, remember it, write it down, do what you need to do! I generally find that once the sail is in that downhaul sweet spot, all I do is adjust the outhaul to keep the sail just off the boom as it gets windier. If it's so windy that I feel the need to yank a whole heap of downhaul on, I just go grab a smaller sail instead!
Sometimes you can get sails to work with the wrong mast, i.e. I used my new 5.6m S-1 last weekend with a powerex mast (note to all the punters, NOT A GREAT COMBO), however a bit of tuning, I managed to get it pretty well sorted for the light winds at the time, but top end range was pretty compromised. Got the proper mast now 
, it makes things work better, but you still need to get it tuned right!
