What are you thinking of doing to make you competitive in club pro as you will need to be about 2-3seconds quicker over a lap, sequential box would cost about £6k which would improve times, pulling weight out of the car will help being circa 1100kg and I suppose theres the aero effect but then im guessing you dont really want to change the way a cossie looks, you could make a completely flat floor with vertical skirts, its cheap, reduces drag and helps with downforce.

Just throwing a few ideas out there for you to think about as the higher you go up the tree the more limitations you will reach with the older chassis design of the sierra.