Stumbled onto this document that answers many of our questions and lists many things we need to know about caring for our cars.
Posting the link and I hope the author doesn't mind

Document extract.
Quote:
My friend made more power, why?
First off, you can never, EVER, compare numbers on a dyno. Not even across the same dyno on
different days. On top of that, I try hard to tell people that it?s actually possible to make a lateral
move on a tune, not even making any more power. That being said, there are only 2 things I can
promise them, and that is a safer and more consistent tune.
http://www.pandlmotorsports.com/misc/ca ... le-1.2.pdf