how much faster than stock...Honestly, there is so much more to it than that. People have ran some very fast times on stock turbos. When choosing an upgrade you should be thinking about EGTS/Boost levels/Towing capability/ Daily driving habits and the ease of doing so/ Ease of installation or lack thereof/future plans with the truck/more fuel or no/Are you in high altitude? if so id stick with the VVT. All kinds of factors play a role. Generally the stock charger will get you to a 550hp MAX level where as the HP levels can sky rocket with aftermarket chargers. I think the 3794 stg2 is capable of 700hp and the 68mm Stg2R i believe another 100 or so. Race times/Faster is just the beginning. Good luck!
EDIT: I run my own meth system, cost me less than $200 to put it together and love it. 44% meth. its an interesting power adder, some like it some dont. Its not like hitting a nitrous button by anymeans.