I have spent lots of time working on the break. I have about 6 different configurations for break cues, and several of them have adjustable weight. I think a few things are definitely true:
1) You can move a lighter cue faster
2) A heavier cue has slightly more momentum
3) A heavier cue has a greater tendency to continue moving in the direction it is pushed.
This makes it sound like a heavier cue might be better. The problem is that if you do not have the strength to EFFORTLESSLY move the cue at the speed you want, then you have to TRY to move it fast. This usually results in a breakdown of stroking technique, and hence less cue ball control. With a phenolic tip, this means more miscues, and more hitting the head ball inaccurately and at a high speed. If you happen to catch the heavy cue just right, you might occasionally get a more explosive break. On the average, however, you will get the best accuracy by finding the heaviest weight that you can swing smoothely and effortlessly.
You can get TONS of spin on the cueball with a phenolic tip. Not as much as a traditional leather tip, but still a lot on the break shot.
I've tried weights from 16.3 to 21 oz. I have a Pete Ohman break cue (which I LOVE). It is 18.8 oz. without any weight bolt. I found this to give me lots of speed and good control. I recently stuck in a 1.5 oz. bolt. This puts the cue's weight up to about 20.3 oz. I find that I get about the same power, maybe a hair more, and a little better control. I am tall and have long arms, so I have a very long strok and a great deal of time to get those 20.3 oz. up to speed. For someone like me, the slightly heavier cue works well. I am not particualrly strong, but have good technique. My fastest break speed on record is with this setup. Also, it seems like with less effort I can get a great POP on the head ball. If you can, you should experiment with adding weight (or taking it away). Don't jusge on the initial results. Usually there is an adjustment process. I found I was initially trying to hit the break too hard-I mean TRYING too much. When I tried to just swing the cue easy and let my technique get the power, my speed actually went up and my control went WAY up. I do think that the heavier weight magnifies the errors, unless you consciously think about hitting the rack "easy".
Hope this helps,
KMRUNOUT