I am going to go ahead and highly recommend Star Power Generator+ and Star Power Nova+ to have the potential to get the maximum number of stars in this mode if you are highly adept at the song (on whatever difficulty with enough notes, though there is definitely the most potential on Expert, which I have been playing on for quite some years) and can take close to full advantage of the 24x multiplier to get, in this case, up to 20 "score" stars plus one more for a "full combo" (no misses or overstrums) for a total of 21 possible on "score-20-able" songs, or one more than otherwise where it is only possible to achieve 16-19 of these "score stars" no matter how you manage the star power! "Maximum starrability" does vary a fair amount in this game, especially in Quickplay+ Power Mode!
These 20 "Power Stars" from score are awarded for a 21.000x or higher average multiplier throughout the song, as available in "More Stats" for the song, and each star below that is 1 1/6x lower in average multiplier in terms of cutoff for it (so 19* is at 19.833x, 18* is at 18.667x, 17* is at 17.500x, and so on) where I have performed in this mode to date. 14* without a "full combo" is at 14.000x in this scheme, and likewise 8* would be at 7.000x... which would put 7* at 5.833x and 6* at 4.667x... this makes sense from what I've observed through my own rather long-time play on Expert in this particular Guitar Hero game!! If you cannot pull off keeping your (usually 24x to fully pull this off) multiplier most of the way to get a "Power Star" score above 14-16 stars on whatever difficulty you play on, it would probably be better to use two powers that gain stars from things other than score that you can pull off, but if you can score "very" well on any given song "normally", then I have long observed and achieved scores above the 15-17 stars possible from other combinations of powers that don't involve score enhancement, from my own use of SP Nova+ and SP Generator+ in my own play a vast majority of the time!