Did an event this weekend and backed to back the hard arms against the softies... I would say that unless you have unusual grip the hard arms are both faster and better to drive. Car is noticably more solid to drive around the track and has better corner speed. I think they should go to the hard arms in the kit and offer the softies as a tuning option.
Also tried out a putty/oil diff. From my quick playing around I found out it is possible to "TUNE" the amount of limited slip effect that the putty has by adding oil. The more oil the easier it gives and also the LARGER the amount of intial give BEFORE it starts to tighten. I prolly went to far with mine so it would be good for stock but not enough for mod. I would suggest starting by lightly oiling your diff and packing with oil. Assemble then turn it over many times to work the oil through the putty. Once it settles simply add more oil if you want a softer diff. I would say start from scratch if you go to far. It takes very little oil to soften the diff so take it easy.