Hmm, your reported 79/80 seems excessive, even for 3 year old jugs...
Perhaps the mechanic needs to test his differential compression rig
against a calibrated orifice...

First, do a bit of investigation...
Physically look up in the carb and see that the butterfly opens all the
way at full throttle... Ya, ya, ya, I know that means you have to
remove the cowling...
If you are not sure where full open is it's actually a hair past
straight up and down pull the bolt out of the throttle cable end and
how it opens then put the bolt back in... Should be the same....
Make sure the heat box flapper is tightly closed... Use mirrors and
bright lighting...
Visually inspect the entire air intake tube for any blockage... ditto
Fly it without the air filter to see if that makes a difference...
Measure the fuel pressure at the carburetor, with an external gauge, at
full throttle...
Check the fuel level in the bowl (a biggie - this is often way off)...
Test run the electric pump with the fuel line dumping into a measuring
can to ensure you
are getting rated fuel flow...
Make the mechanic prove to you that the mags are all timed at 25
degrees... are the
mags roughly at mid range on their adjustment slots - if all the way to
one end you could have a worn acessory gear
Verify that the impulse cam (spark retard) is releasing after the
engine starts...
What is the difference in static rpm on L mag versus R mag?
Verify that the prop has the correct pitch... another biggie

If nothing is found above, then rig up a dial indicator and measure the
lift on each valve, could be illuminating... I wouldn't worry about
oil in the lifters... Just do each measurement 3 or 4 times... Call
Lycoming, they can give you the lift specs...
If the cam looks OK then pull the rocker arms to:
1. See how the bushings look.. If egg shaped you are losing lift and
power... Don't let
some yahoo try to tell you that the hydraulic lifters make up for this
- they do not!
2. see if the valves are free... WIth the rocker out of the way you can
use a lever to
depress each valve for sticking... The exhaust are the ones to be
suspicious of... If any doubt rope the cylinder and remove the valve
spring and see if the valve is free...

If everything mechanical seems up to specs then squirt solvent oil down
each valve stem and work the valve up and down (WD40, Mouse milk,
Marvel Mystery Oil, etc.) then change oil for a fill up of lightest
weight oil you can get leave the filter as is add a can of AVBLEND
and a pint of MMO (Marvel Mystery Oil) to the fill up, and go fly the
heck out of it for 5 hours... See if that solves it...

denny