I'm using an upgraded F5A51 (same as W5A51). I bought the clutch pads replacement, stronger wavespring and seals from Import Performance Transmissions (I think you can also try this shop called level ten). I also added an additional trans cooler - can't remember the brand though. I think the rebuilt gearbox is great value for money and the additional cooler should make the gearbox last longer. However, you might need to rebuild a torque converter to complete the set up. I am using a stock torque converter presently on my upgraded gearbox and I think it can take about 48kg-50kg max of torque before slipping. I think heat is a key problem to slipping as well.
However, having said all these, nothing beats manual!