It wasn't 8000 of the most powerful sith, just the survivors of Ragnos's fall.
There appear to be two kinds of TP applications; those against individuals and those against large populations. The former's most impressive examples are probably Lord Nyax and UnuThul almost dominating Luke. The latter probably goes to Vitiate and Ziost, although Palpatine's Coruscant mindwipe might be more impressive on a refinement level.