The cam's not a huge issue, it will probably do okay. What you will get is a slightly rough idle and less low end response. It will feel a little soggy when you stab the pedal from a stop. Anything above 30-40 mph the effects are less. Your mechanic's not doing you wrong, just sounds like he's from the older school of thought. In today's world of low compression and crappy gas, "putting a cam in it" just doesn't get you much. Drive it around and see how it is, try to advance the timing as far as it will take without pinging.