My experience with Gear Vendors is based off of my Father's Challenger.
It functions well and certainly serves a purpose, however I do feel the "gear-splitting" function is more of a marketing term rather than a matter of practicality. At least the way the GV was executed in the car, it was based off of an additional headlight dimmer switch, the on/off function of the button made it very cumbersome to try to gear split. In turn, it just operates as a 4th gear.
It's very tough to beat a manual overdrive setup in a muscle car. The TKO's are not well known for their shifting quality, but I feel many of the problems that arise with those swaps are caused by the vendor, not Tremec. I purchased a 6-speed T56 Magnum from SST and, so far, have no complaints with the vendor or the product. That is far from a "no-cut" swap though..