exhaust pipe diameter for a mildly modified 340

I thought a street engine needed some back pressure to keep lo rpm exhaust velocity up for low speed power?
Isn't that why street driven engines benefit from a small cfm carb and why the factory reduced the tail pipes
to 1 7/8" from 2 1/4"? This wouldn't apply for an open header race engine at 6000 rpm.
....but the question is how much power might be lost going from 2 1/4" to 2" exhaust? I'm wondering if there
may even be an improvement below 3500 rpm?