what clutch to use
 

hi my car is a sti 2.1 1997 running 451bhp/425lb 6 speed 5 speed clutch fly wheel starter.fitted greenspeed 4 paddle clutch they told me it would be fine ,but on the motorway in 6th put my foot down it is fine but start to drive hard ie 6th to 5th at speed it starts to slip. what power and clutch do you use ?trying not to spend to much, as money started to run out thinking of a 6 paddle helix .