The carbon arms are not an "upgrade". It all depends on what your car needs. All of the carbon parts are to reduce flex. They are stiffer. So carbon towers wont flex as much as stock towers, there is a down fall though, they are more brittle due to the increased stiffness. I ran a carbon chassis and towers, everything else was regular plastic, it is the way I liked it. I tried the carbon arms and did not like them. Don't remember the effect they had on the car. On my Centro mid motor conversion I had the carbon front brace and front arms but left the rear stock. All of my 5 series vehicles use stock plastic parts.

I know the shocks are expensive, but they are worth it. Once I put the shocks on my B4, I bought them for my T4.1, my Centro, my B44.2 and my Prolite. The springs started to add up after that. Now I have plenty of springs for my 5 series of cars and trucks.