I'll admit this may be a stupid question

Is there any benefit to be had from a 3" turbo back exhaust when the o2 housing outlet is 2.5"?

 

Bigger pipes are better flow. From a piping standpoint if you had a 3" exhaust with a 2.5" cat back to a 3" exhaust, it would flow faster than a full 2.5" exhaust.
Isn't gonna hurt as much at 300hp as it would at 500hp.

if you think about it, your turbo outlet isn't as big as the rest of your IC piping.

I'm sure somebody will have a better thought for you, but I work with a Kerosene flow bench. larger piping is less restrictive and sees higher flow rates. but a slightly smaller SHORT section (especially at the beginning) isn't that damaging to the flow rate

 

There will still be less flow resistance on the outlet with 3" piping versus 2.5" even if the outlet isn't 3".

 

Short answer. Yes.

Longer answer.

Think of it like this. In terms of restrictiveness, if you simply removed your exhaust from the O2 housing back, would you see an improvement in flow? More than likely.