If the "tappet" noise has only developed since the service I would be leaning towards the oil that they put in.
However at 100,000 miles then there will be some wear and tear on the engine and if the noise is tappet/cam/bucket wear then this can turn out to be an expensive fix.
Upgrading the heads on a NA engine will give you very small beneficial power returns.
I do have spare low mileage cyl. heads in stock complete with valves, cams, buckets etc.
If I can help further then drop me a pm.
Alex.