I'm sure the catch can helps fuel economy, but only by a minuscule amount. I'd say your gains are due to seasonal/elevational differences.

But since we are on the subject... I've been wanting to get a catch can but I refuse to pay $180 for one. I found this how to:

Home Made Oil Catch Can Photo Gallery by AZ Solo at pbase.com

Looks neat, and the best thing about it is that the catch is clear so you can see the crud coming out. Looks like it'd cost about $40 and looks cool.