I know this is a necro, but I had an idea or two that might help to balance this idea out... what if the StarBeat Power didn't directly affect your score, but instead affected the Energy Gauge/Life Bar -- you fill it more quickly and lose it more slowly while in a groove, and every note you hit adds to the gauge for it. It would be more balanced than even saying that StarBeat increases your score by even as little as 6%, and for the less skilled among us, it could be a helpful tool that lets us improve with just a little more room for forgiveness as we start tougher maps.
Another thought is that StarBeat could be a mod sort of like HR - more precise timing and the ability to gain bonus points, but impose a starting penalty on points, so that playing SB and not using your gauge won't provide any benefits. The precise numbers could be tweaked, but maybe a 20% base penalty (so a FC worth 1,000,000 normally only delivers 800,000 if the gauge isn't activated), but maybe allow a 40% bonus to points earned while in a groove. This would make notes earned during StarBeat worth about 112% their value on the map without the mod. Of course, some issues like working out a perfect play with SB on to only about 112% would have to be factored in, as well as how quickly the SB gauge fills and how much of a song can potentially be grooved through SB in order to average that sort of number.
I think the first idea would be a lot easier to balance, and I doubt it would suddenly bring newbs into the Top 40 or otherwise drastically impact the scoreboards just because they can now complete songs just a bit beyond their current skills. It could even still have a penalty, maybe 6% or something.