There is no exact scientific way to say which weigh you should use.  It is all trial and error.  Changing the weight will not normally affect ball flight numbers but it will change how the club feels throughout your swing which in certain instances can help you gain distance/control because you are now able to release the club properly.  

The best way to test it out is to use lead tape and try different amounts to see what feels the best.  I'd recommend trying it over at least 2 different days to confirm what weight feels best for you before ordering the new weight.  Sometimes one weight feels good one day but a day later doesn't.

Good question, however it comes back to Swingweight vs anything else.  The lighter the swingweight (using a lighter sole weight) the faster the club can go and thus gain of yards however there is a trade off that your swing may go to fast and you cannot "feel" the club each time for a consistent swing...

IMHO

Check with a fitter in your area and he will tell you the exact effect they will have on the club.