I have way more distance with lighter weight shafts, unfortunately my distance gain is either left or right of the fairway.  I prefer the heavier shafts.  I just seem to be more consistent with the heavier weight.

No, a lighter shaft does not always mean more distance(clubhead speed), if the shaft you are using is too heavy then a lighter shaft will mean more distance.

Trial and error is the only sure method I know of to determine the best shaft weight for you.

If you go too light you lose all feel of what you are doing, thus you spray the ball all over the place.

Usually the torque on the superlight shafts are higher so if you have a smooth tempo, sometimes you can benefit from going to a lighter shaft because you will be able to swing it a bit faster.   If you load the shaft or have a quick, fast tempo,  a superlight shaft like that may change your timing and not allow you to feel the head and a heavier shaft will probably work better for you.

I use a 86 gram x-flex in my driver. and it hasnt slowed my swing speed down enuff to complaine. i still bomb it round the 300's during a smooth swing. but i have always had a fast swing. I tryed a 55 gram once to horriffic results. it just felt so whippy i generated way to much spin. 70gram is the absolute lightest i can go in a driver.