You should be so lucky on the shims, nissan don't make them any more...first thing to do is take the rocker cover off and see what the clearences are, sometimes the oil spray bars block and the cams get excessive wear on them so check the cams and rocker pads for scuffing and abnormal wear and witness marks, comp tests pressure wise are a bit meaning less as static cam timing will make the results vary, your looking more for similarity in figures. The only real test is a leak down test.