unlock convertor have serious slipage. say 20-30% diference between engine imput rpm and imput shaft rpm(the shaft that is driven by the tc)
haveing the TCC brings the slipage to less then 50-100rpm between the two items. thus drasticly imporves mpg
peformance wise. example if you are shifting @ 3000rpm and have 30% slipage in non lock up you shift speed will be 5-10mph lower then if you had a lock-up conveter. also haveing the TCC engage loads the engine sooner as well