Consequences of a Botched Valve Adjustment?

Just curious, what would happen if this was done completely wrong? Power loss? Damage or excess wear of cams and valve stems?

Depends how you botch.

Excessive clearances will lead to increased valve train wear, mushroomed tappets etc. & loss of power.

Too tight & you will burn an exhaust valve.

As clearances open up you can hear it, & do something about it.
Too tight is the one to advoid every time.

If you've ever done the clearances on a VW beetle the 911 is really not too different,
nothing to be scared of. The Pelican article explains the procedure very well.

Regards, JG