Considering your using a very fast powder and also the ONLY powder for the 458 Lott IMO, I doubt that you will lose much velocity...I would venture to guess that you will get 2200 or better..I think IMR-4320 will burn completely in 16 inches...but only a chronograph is going to answer your question as I have seen as much as 100 FPS deviation between two barrels of the same caliber and all things equel...