Joe, mine doesn't slip out of overdrive as much as it unlocks the torque converter. I had considered trying to hack the TCM so that you could flick a switch on the dash that kept the TCC (Torque Converter Clutch - the locking mechanism) in a locked state but considered it to be a little too dangerous.
It WOULD be nice if, once the TCC locked, you could put your foot down a little harder than we are able to at the moment. However, I was watching the ECU's reported "Engine Load" figure while driving along with the van behind and it wouldn't unlock until the load figure passed 70%-ish. I don't know if that figure can be adjusted to 85-90% so you could give the thing a bit more of a squeeze to maintain speed.
I do know that when the engine load is 80% or higher, the engine temps rise dramatically. In order to be successful, you'd have to (in order):
1) Improve the cooling system to a significant degree
2) Probably chip the motor so you could HAVE extra power
3) Make the TCC changes
There might be some advantage, if you could get enough hp & torque out of it, in changing the diff ratios. You'd then force the TCC to lock at 1300rpm instead of the current 1600rpm, and have the thing cruise (100km/h) at about 1700rpm instead of the current 2,000rpm.
I suppose you could buy a 550, which already does that, but if the changes to achieve the above weren't too expensive, it's somewhat appealing.