catalytic converter question

A bit higher up, someone asked a question about taking the cat off. Of course he
could send it to me...

I have a different problem.

Since my car ALMOST didn't pass the CAL emission test  (it did the 2nd time, by a
molecule) 3 different people said I should replace the Cat.

My question is, will the gas mileage change? In other words, if I have to drive
it
3000 miles, and it gets (for example 15 mpg) that's 200 gal @ $2.25 average =
$450.

If I suddenly get 3 mpg better, then that's 166 gal @ $2.25 =$375.  So I just
saved
$75.
On a 6000 mi round trip that's $150.

Forgetting the environment for a moment, and the all electric 200hp engine that I
have still to invent (which charges up on residual moonlight while I'm sleeping)
Should I bother with the catalytic converter?

Capt. Purple