Graham,
This is a slightly contentious issue. You will get many varied opinions so I'll just give you mine.
The bushes used by Porsche are fitted in to the wishbones at the factory and, because the bushes have internal voids to aid suspension flexing the orientation of the bushes is critical to their correct operation. For this reason the bushes are not made available for replacement "in the field". The official line is that the complete wishbone is changed. It's a relatively simple job from a labour viewpoint but will require a suspension re-alignment once new wishbones are fitted.
Because many people felt it unreasonable that the whole wishbone had to be changed a representation was made some while ago now to Powerflex with a view to them making a suitable bushing kit. This has, indeed, happened although I believe there are some restrictions as to who is allowed to sell them. I don't believe Powerflex themselves will supply direct. The Powerflex bushes do not have the internal voids a la the Porsche OEM product and therefore have no restriction on their orientation. However, according to the advice I got when mine were replaced, the labour involved in getting the old ones out and fitting the new ones is not insignificant. All very well if you are planning to do the work yourself but if you are paying someone else to do it you may finish up with something very little less expensive than changing the wishbones complete. I had JZ Machtech do mine and they recommended the wishbone change and it turned out very little different to their quote for the bushing change.
If you decide to go the Powerflex way, make sure that whoever fits them knows the "wrinkles" - failure to put enough lubricant on the shaft through the bushings apparently gives rise to longer term "creaking" in the front suspension. As long as it's done correctly the Powerflex bushes are slightly more stiff than the Porsche items and give rise to a slightly more direct feel in the steering. You might find some more detail if you search the forum for "Powerflex".
Bear in mind that the alignment is required either way.
Hope that helps.
Regards
Dave
