The single largest area that needs work on a standard F109 is the side springs. It is also the area that seems to be the most contradictory when reading online information. Some seem to get the car to handle fine using the standard kit or option springs and others need to make serious modifications. The only pattern I can find to this is that most of the people making the standard springs work appear to run foams on carpet.
My view (and I've discussed this with others and they agree) is that the original side springs are simply too soft to support the pod when turning under power. The springs "collapse" for want of a better word. They are also too soft to make the pod return to center when rolling from side to side. This makes the car react as if it is tweaked and causes the car to spin out far to easily. I was able to use the hardest option spring and a bunch of shims to put a lot of tension on the springs when I started looking for a solution but in the end I knew I had to find different springs.
Most seem to do the CRC spring mod.
However, CRC parts are not the easiest to source for me as ordering from the U.S tends to be quite expensive if you want any form of express freight, and not ordering express can lead to waiting for months for an order to arrive. I was able to source Associated springs more easily, though this meant I had to devise my own mounting system. Unfortunately I didn't take any pictures at the time but its turned out to be quite simple involving 2 5mm ball studs and small amount of cutting to the original spring seats. This allows me to also adjust pre-tension and therefore tweak. It also doesn't require any modifications to items such as the upper chassis pivots.