I'd either go with a stock rubber, or a nylon bush- the powerflex ones have broken a few times on Andy's car, and I've ripped a set of superflex ones.
Few people on CP seem to have had problems due to bad installation- to be fair to Powerflex they did send Andy a new set when he told them, and both our cars are running quite a lot of power with sticker than normal road tyres.
I've got some Powerflex ones if you want to try them, £10 - PM me, I'd not use them on my car as I've got Nylon ones.
Cheers
Pete