the 6's aren't restricted by the top speed limit at 186, but instead they are limited by terminal velocity (a.k.a. drag, a.k.a. not enough grunt to go any faster). If you re-gear them like mine, then you will become limited by the gearing (you hit redline in 6th before the top speed limit at 186). There is a benefiet to a TRE on a 6er though. The power in the lower gears is reduced through the timing retard that is in the factory ECU. So it makes the transition through the gears more seamless, and gives you more power at part throttle while in the lower gears.
I am not sure whether or not the timing retard exists in the lower gears while at WOT though, it may go away there. Someone else who knows could chime in on that. But I can tell you that with my homemade TRE I get consistently lower 60' times at the drag strip (not crazy lower, but a little lower), but I don't really see any changes at the fast end.