When I say bad converter what usually happens is a problem in the stator where it has no stall speed anymore. Seen it before and on a stock converter it makes the car feel like a clogged exh as when you hit the gas it has no rpm since no stall and it feels like a dog until the car gets rolling. Then you can hit it from a 40 mph roll and it may feel ok but from a stop it would be a pig.
Now if a lock-up converter would lock-up way early in a low gear it could feel sorta the same but the lock-up would have to be unlocked at idle or the car would stall at an idle. They can fool you to look at other things like the eng for a lack of power. Course a clogged exh system can also feel almost the same when it wont let the eng rev up much. Ron