Tight valves will affect the idle as well, by allowing a little bit of compression to leak out.

A quick and fairly accurate way to determine if all cylinders have even compression is to pull the coil wire and crank the engine around several times, listening to how the starter labors over each compression. If there is some problem you can tell that one or two compressions are easier than the others, and bother to get a compression tester to further narrow it down.

As a bad cam lobe wears down it will first affect the cylinder at higher power settings, as it can breath adequately at idle (even better really) through the reduced valve opening, but falls behind at higher flow rates.