I did my own test on my DD Rodeo, 2.6l.
W/out cat my mileage per tankfull increased about 25 miles, while the top wot speed increased 10 miles.
I think what the cat is doing is burning the small amount of hc left (don't really know how much) in the discharge.
Also used to remove egr circuits on bikes (F-3 and F-4 mainly) which added about 18-20% increase.
So I use a hollow cat, with a tube trough it, here in Cali is expensive to be cough tampering with emission related components.
I had run leaded gas on my Swift GTI, which had no impact on either performance or economy...
Just my .2 cents