Bump steer is caused by the angle of the steering arms on a lowered car - they point upwards instead of flat so when the wheel moves upward the steering arm is already higher up the arc and so you get an effective shortening (or lengthening, can't imagine it at the moment) of the arm which will toe in or out that particular wheel. You will get bump steer on a lowered car irrespective of what control arms you have fitted. Elephant Racing do bump steer correction kits that will again restore the angle of the steering arm to the same as stock.
The lengthening of the pin returns the angle of the control arm to a flatter plane ideally the same angle as it would be at stock ride height. If you don't lengthen the pin the arms will point upwards and the pin of the ball joint will be rotated outwards so under heavy braking or cornering there is a risk of the pin rotating and touching the rim of the socket at the end of the arm. This will eventually result in either a sheared pin or a fractured arm, whichever gives up first. They also do a 19mm dia pin kit (stock is 17mm) intended for racing to make the pin more robust for when you are riding kerbs. Clearly you need to have the ball joint aligned correctly or you'll have the problem I describe above. However this means you need to drill out the holes in the hub carrier to 19mm.
Yes the dimples on the pins can be a weak point. With the Rennbay kits they don't machine a dimple but a groove around the whole circumference of the pin which, though difficult to believe as you automatically think you are weakening the pin, actually relevies stress concentrations.
The geometry correcting kits with lengthened pins do look a bit disconcerting as they look too long and slender, but they are very strong. I saw a photo of a car that had a drivers side head on collision and the lenghtened Rennbay pin was bent nd the Control Arm had acutally snapped about 4 inches inboard of the ball joint, so the pins are stronger than the arms.
Also with the Rennbay kits you can get them with Phospher Bronze bushes instead of Nylon ones as with stock kits. The Nylon bushes don't last very long at all and break down and you end up with the steel ball grinding into the alloy socket. The phospher Bronze bushes not only tighten up the socket but will last. They also have grease nipples so you can renew the grease inside the ball joint at service time.
If they do the Rennbay kit for 968's i'd heartily recommend them.
Anyway, this is the sum total of my knowledge on this subject that i've accumilated over my extensive research ahead of installing the KW suspension on my old 944 turbo (sorely missed!!). With KW suspension, RE solid top mounts, Rennbay geo correcting ball joint kit and 968 castor bushes the car had really responsive and tight steering feel. It was a massive improvement over stock and felt much tighter than a modern sportscar - even a Boxster. It was a joy to drive.
I'm sure there are some holes in my understanding, but I did research it and asked may people directly who have had good and bad experiences with lowering these cars. Short of upgrading to Charlie Arms or RE arms, this is about the best you can do with stock control arms to minimise the chance of them failing if you lower the car. I don't believe this problem is as prevelant on the 968 as the 968 arms are considered an upgrade on 944's, but if you start lowering the car you need to look into these things carefully.
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