As for Low 2-C Mario casts, it should be worth noting that "the power of the stars" that Bowser gloats about acquiring is also, by a fellow Luma, said to be gifted to Mario by the young Master Luma. Whether this "power of the stars" was just the spin is still up for debate though. This could potentially justify a Low 2-C rating for Mario when gifted the Power of the Stars by the Luma since it could directly combat Giant Galaxy Bowser who's Low 2-C with the Power of the Stars (because of the Grand Star) he obtained. And it should be noted this isn't base form Mario, and that just because he's beaten Powwr Star users in base doesn't mean he scales to their full potential, as a Grand Star, Big Star (Bowser has this in 64 and claims he had the "Power of the Stars" there), and many Power Stars (as seen by endgame 64 Mario) are needed in order for you to be able to fully utilise this "Power of the Stars". Hell, the name even implies it's plural, and since a Grand Star and the Big Star are proven > a single Power Star, it's fitting IMO. And the Lumas all sacrificing themselves are like multiple Power Stars (since they can become them) being used to combat the Low 2-C Black Hole, aka the Power of the Stars, a Low 2-C force being used to revert a Low 2-C event. And Master Luma is considered special and should be considered special, as he's the only Luma who directly aids Mario in fighting as well the evidence from the pics below.
Lumas stating that Master Luma could help Mario use the Power of the Stars
Lumas making the distinction that they can only help "a little" when in comparison to what Master Luma could do, showing he's a special case among Lumas as well
Given Luigi, Mario, even Bowser, and Rosalina (in 3D World) could use the spin and canonically have access to the Power of the Stars, this would obviously scale to them. Bowser Jr. via Grand Star-powered mechs would also stay at his rating. Anyone who's never used multiple Power Stars at once, a Grand Star, or the Big Star likely wouldn't scale. However, luckily Wario and Yoshi do gain access to several Power Stars and share the same power soirce as Mario in 64, so they likely had the Power of the Stars in 64 as well. Peach is arguable and DK never had any of this so he'd stay where he is from scaling to Base Mario.