I ordered my ULA w/a 24". When it came time to replace the barrel I went to a Pacnor 25" for improved balance.
There was no noticeable velocity increase using the same 140 TSX/Varget combination. Accuracy, on the other hand, went from 7/8" to 3/8". Nothing to do with barrel length, just an exceptional barrel for a hunting rifle.
My load chronos at an average of 3008 fps @ my home elevation of 6803'.