My two cents on this subject is that if you change ball weight you should see no difference in speed and revs (revs only if you're using the same ball drilled the same way as different balls and drillings will affect your rev rate). However you will see a difference in kinetic energy and hitting power because of the difference in momentum produced by the differeng weights. A heavier ball will have more momentum and kinetic energy than a lighter ball, which is basic physics, however rotational kinetic energy can change based on drilling and core design. It may not be an overly significant difference, but as a general rule a heavier ball will have more power behind it at impact than a lighter ball.
As for the muscling aspect of it, switching to a lighter ball will magnify any muscling issues you had already because you're muscles will be going like you are using the heavier ball, but won't be working as hard to support the lighter weight. The end result of this is that you will impart greater speed and power at release which will decrease results at impact. My observation on this is based on the video I posted up at the start of last season (2007-2008) where I was told by numerous people on this board that I had a controlled (muscled) armswing. It took me almost a full year to figure out how to correct this problem, but once I did I noticed a tremendous difference in not only accuracy, power and control, but also I wasn't as tired, my forearm wasn't as sore and I had much better balance. In that time I used balls ranging in weight from 15lbs to 16lbs and I noticed no difference in speed, control or balance if I was throwing correctly.
Dennis: I'm having a couple issues similar to yours where I hit a 230-250 game for the first two and then bomb to 190 in the 3rd. I did that this week in league, though I know why: a couple times I missed inside leave a 6-7-10 and a 4-6-7-10, and then I made an incorrect adjustment.
15.5lb MoRich Nsane LevRG (medium and medium-heavy)
15lb Roto-Grip Cell (heavy oil)
16lb Colombia White Dot (spares and extreme dry)
16lb Roto-Grip Neptune (light and dry)
High game: 300
High series: 779