"He said the Cam timing should be right on the mark or a little bit retarded. "

Right on the mark, yes. A little bit retarded, NO. Where did this fellow learn auto machanics? This may well be the reason the car is exhbiting a performance loss, i.e. the cam timing is off. Also, valves getting tight with a worn cam does not sound entirely correct either. If anything the valves would be likely get looser.

I would make that the last visit to that mechanic.