I would look at how the heads flow first. Then decide how much compression. A good cam with lower flowing heads can perform better than a good flowing head with to much cam.

You might even try a small cam. Seen some what you would consider small cammed cars run hard.

 

Those numbers and times are very low. I think your problem is that you are running a cam that looks like it is designed for ls1 heads and that you have ****ty headers. also 167,000 km is definately not alot of miles!