the MT643 is an excellent transmission.. not only do you get a lockup converter , it is a beefier more heavy duty unit, so its more condusive to heavy loads, towing, and mountain travel..
it isnt any "faster" than an AT545 as like the 545 its a 4 speed with a 1:1 ratio on the top end.. non overdrive..
it shifts a tad rough for some people because it is heaby duty, doesnt bother me as i like that feel..
it definitelty doesnt make near the amount of heat that a 545 does..
ive gotten rid of 2 AT545s and one of them became a 643.. it works much better than the 545!!