I took the cover off my engine to look for the PCV valve. It was late so I didn't bother to put it back on. I think the engine runs cooler with it off. And my milage seems to be better.Is this possible or is it just in my head?

run cooler? yes
better mileage? no

What are the pros and cons of running with the cover off? any others?

how the heck are you supposed to gawk at ur m90 with the cover on. 1- con

I am stuck in L36 World.

yeah but the pros and cons are about the author. he has an ssei

The only other con is increase noise......but why would you want to muffle your whine.
Pros outweigh the cons.

Leave it off and enjoy.