I have the UM Haldex flash. It wasn't a night and day difference for me, because you really only feel the effects of the AWD when you are pushing it a bit. I emailed back and forth with the main guy at UM (name escapes me at the moment) and discussed the features of their software.
Here is my understanding of the features. The UM software is similar to the stock Haldex software in that torque transfer to the rear is variable. The algorithm is just a lot more aggressive than the stock software. It is based on a lot of inputs like throttle position, speed, steering wheel angle, etc. The UM software gives large front wheel bias while cruising to maintain gas mileage, but will become aggressive under throttle or while in boost. It also keeps the Haldex clutch engaged under hard braking like the HPA comp controller.
I'm not an expert on the HPA Comp controller, but the impression I got from the forums is that its a more "full time" neutral bias.
There will never be rear bias with any of the Haldex controller products. The theoretical max output to the rear is 50% of the power because the front wheels are always spinning, even when the back is fully engaged. It's not gonna drive like a RWD car. When the rear wheels are engaged it will understeer less but you are also not going to oversteer through every turn. If you really want the full effects of your Haldex controller to shine, do the ESC defeat with VAGCOM and if you're still on stock rubber you can four wheel drift from a standstill through a u-turn. I was most impressed with the UM software this winter when I finally got to take the car out in the snow and the AWD engagement behavior was completely different than the first time I took it out back when I was still stock.
With the stock tune in the snow, I would turn into a slide, and at first the car would understeer like a mofo, and once it sensed the understeer for a bit, the back wheels would eventually kick in and send the back end around.
This year, with the UM flash, the power transfer was faster, smoother, and more seamless. There was no "understeer period" at first when turning into a slide - the car just slid wherever I wanted it to go.
Finally, if you want to hear from a reputable group that uses the UM Haldex flash on the track, I believe Tyrolsport has it on their Golf R. They may be active over on the 'tex - try sending them a PM for their feedback. One of the things that convinced me to go with the UM flash was a post by TS when the switch from the HPA comp controller over to the UM flash and commented that they didn't really notice any difference between the two on the track (or something to that effect).
But don't take my word for it, I definitely recommend running some searches on vwvortex and get others thoughts. I posted a lot about the UM flash over there in some of the threads too (my user name there is Super .:R-Type)
