What does replacing these parts do?
Bump steer kit eliminates just that, bump steer. Here's the description taken from Whiteline website: "Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the road or track interacting with improper length or angle of your suspension and steering linkages.
On the VW Golf and most modern cars the front of the tire moves outwards, as the suspension moves upwards through travel, a process known as the front wheels "toeing out". This gives roll understeer and induces an underperforming handling vehicle. Huge amounts of toe out were experienced during testing on the VAG platform which lead to the development of this Whiteline bump steer correction kit part # KCA314. To achieve zero bump the front tie rods were developed to travel on the same arc as the suspension when the car goes through travel. Simply its matching lengths and arcs to prevent any unwanted steering of the front tires."
So if you feel your car steer in one direction or another when your suspension travels, more than likely it's bump steer you're feeling. I can feel it the most on the freeway...if the kit doesn't sell, I'll just put in in the car and use it because it does help. However if I do end up selling the car, I'd rather get top dollar for this kit since it's never been installed.
http://www.youtube.com/watch?v=XTxP__nIeRw
Rear trailing arm bushings just stiffen up the rear end because OEM bushing has very large air pockets in it. I upgraded my front bushings and it makes a huge difference.