Cylinder Head decision

If your are sticking with the mild 318 long term, then a compression increase is certainly going to help the torque at the lower RPM's, particularly with a mild cam that has an early closing point. You could also achieve some better lower RPM torque by advancing the cam. But the increase will be limited with the 318's low compression height in the pistons (the tops 'down in the hole').

But the results will be questionable if the bottom end does not have good rings and compression as said.

Do you know what heads are on the car now?

I would not bother with a cheap valve job on the 920's; with a couple already recessed, you can count on putting seats in all chambers and had better put in t new guides to make sure things are sealing right or is can become a waste of time and $$.

I can't see how the CME heads are any particular type, open or closed chamber, from their ad that you attached. So how do you know what you are getting for that low price?