The current build (which may or may not be tweaked before final release) would allow for Time Lords as low as Level 44 Superbosses.
I'm toying with an early Tetragrammaton mock-up which is a Level 47 Superboss. However, I may chop and change a few things with regards the relationship between Time Lords and High Lords - splitting them into different tiers.
True. Looking at the table, Level 47 translates to a maximum of Star sized.
However, I may move High Lords out of the Eternal tier and into the Supernal tier whch would mean they could indeed be bigger.
I should point out that (and I'm sure it will fall on deaf ears*) I don't suggest overuse of the superboss concept. The stat-blocks are hypothetically ginormous. Tetragrammaton alone is probably going to be 4 pages himself. Its virtually like a mini-adventure for the bigger superbosses.
So my suggestion is to only use superbosses once every 2-3 levels of play.
If solo monsters are the 'chapter' bosses, then superbosses are the 'Act' bosses.