a l/u converter has a clutch in it that the tcc sol engages it at cruising speeds for better gas mph. It is just like a regular clutch only electronicly controled and applied. If it stays locked you would feel the eng. being loaded down (lugging) not appling would give worse mph and can lead to...
shift kit and wide band are only a band-aid and not even that if the rest isn't right. it sounds like him changing your VB wasn't the answer either, I hope you didn't let them keep the old one. mix matching vb's and govenor's is gonna get you in trouble unless they knew what they wee doing...
too hard to say... it would be a guess.... I had one very well built and it snapped the fwd drum shaft first pass. a good build w/o breakage still kinda hard to guess, but 50k would be a reasonable one. or do like salvageV6 did and have a little more assurance.