From my experience on my truck, the WB with and w/out cats (post cat and turbo), it was only about .1-.2 difference, which I attribute mostly to increased air flow.

 

I'm thinking wrong... if you put the sensor at the tip of the exhaust system it's .5, like you'd do with an LM-1 and the exhaust clamp.

 

That's what i heard like i said earlier. Now putting it on the tail pipe, that could be farther off i would imagine. But if it's only .2 off, not too big a deal, unless you are FI and you want to get it absolutely perfect.