Wow, my first post on TDIClub in nearly two years!
I can tell you the rational for switching out the twist beam axle was based mostly on durability. We've tried a number of different methods to strengthen that rear beam. Tried the VWRacing method, tried custom plates to give more triangulation (though the fuel tank limits the amount of triangulation you can do), etc. None of them really hold up to any kind of lateral impact on the rear wheel. And by impact I mean something as simple as sliding sideways and hitting a baby head sized rock that's been pulled up by a AWD car higher up in the field. The arms trailing back from the bar to the hubs are just too long and provide too much of a torque moment. I'm not expecting a rear suspension to hold up to slamming into a curb sideways at 50 mph, but I am expecting it to hold up to what I consider average conditions for the course.
Racing, rallying, daily driver, I don't think anyone can argue that the rear axle holds up particularly well to any kind of side impact.
I dealt with the problem for a number of rallies, and started getting tired of having to deal with a crabbing car on stage. For example, the passenger side rear in the above picture is pretty cocked up. It makes the rest of the stages a bit uncomfortable, then ties up your next service swapping out the axle. Talking with one of the service guys for the previous owner, I understand he went through his fair share of axles as well.
The last straw was the fourth time it happened (in as many rallies), during the 2009 Rally of the Tall Pines. Midway through the rally we were a bit wide coming over a ! R4 over crest, I think. Barely even touched a rock on the drivers side with the rear. Definitely not enough to upset the car. However, it bent the axle and as a result it ripped the rear shock top mount out of the chassis. At that point the handling becomes significantly worse than any MacPherson setup you can think of.
Had to run like that for > 30 km. Get to service, didn't have a spare axle, so just ended up bolting the top mount back to the wheel well. Head back out. During the next set of stages, we must have hit something else hard enough to do the same thing to the passenger side. Bent the axle and the shock top mount ripped out. Did another 30 km or so and it was just as much fun. Got back to service bolted it up and kept going. We ended up finishing as fastest 2WD, and 11th overall (out of 51 isn't bad at a Canadian National)[1], but we were almost a minute back on 10th place, and I know that some of that was due to trying to keep the car on the road when it really wanted to go sideways.
After that I realized that it doesn't matter how awesome that twist beam is. I can't have it continuously failing on stage like that. In rally (as in most things) it's just better if things don't break while your using them, no matter how cheap or easy they are to fix. The Neon suspension more robust (for rally for sure), a heckofa lot easier to service than the twist beam, and works for the SRT-4 Neon rally guys. Oh, and it has the same bolt pattern as the A4. For rally, the setup has really solved some of the critical weaknesses we experienced with the stock twist beam setup.
Haven't had too much run time on the suspension. We ran it at the 2010 Sandblast, albeit with Monroe Sensatrac struts from Autozone. They didn't even last the whole rally, but we didn't have anything else in time to put on there. From that run I learned that the Neon rear suspension won't kill me. The next event is all tarmac, so that'll be the first real test of how the car as a whole handles and whether we can dial it in appropriately.
[1]
http://rallyracingnews.com/cars/tallpines09-results.html
[2] Craftsman hand tools are great, you can get lifetime replacement, and Sears is everywhere! Until one breaks at a service in the middle of the woods. Good luck finding a Sears then.