In the service manual there is a specific procedure and special tool that is used to check the lifter to see if has issues. Playing around with different shims won't help if the lifter is bad, or if the rocker arm has wear. I would suspect worn rocker arms versus a bad lifter, but you never know. The tool measures the height of the lifter and either gives it a + or a - value. I would be very leary of playing around with shim thickness. The M100 motor is very expensive, and the wrong shim could cause undue wear or burned valves. If this was my car I would remove it from this shop, and find someone who has experience with M100 motors. IMHO.