All non lockup converters slip at highway speeds.
Higher stall speed off the self converter will slip more. You will hear the term "loose converter" when addressing this condition.
So if you have a 3000 rpm stall speed converter and it slips 10% at cruz'n speed you losing 300 rpm at speed.
My engine will turn 2900 rpm at 60 mph with 3:55 gears because I have a higher stall converter.
On the other side of the coin, I find that hi performance engines that are coupled to a higher stall converter easier to tune at idle because the are slipping and do not place as much parasitic load on the engine.