I installed a extended and ported Barstow barrel in my Glock 23, 40 cal.. With 180 gr. bullets, the point of impact was lowered about 2" @ 10 yards. The 155 gr. high velocity loads went about 5" low at the same distance. This was extremely annoying because I had just installed expensive Novak night sights. Do the barrel first, then get adjustable sights if you intend to go back and forth with the two barrels. Use a sight that has repeatable elevation settings, so you don't have to go through a major sight in process every time.
Jeff