On a otherwise well optimized h/c/I LS build how much power will be left on the table if using headers with to small primary tubes? Big difference between say 1 5/8" to 1 3/4" and 1 3/4" to 1 7/8"?

 

In general, I see a bigger difference in the midrange torque moving to larger headers than I do in the top end power, unless the motor is moving enough air that the primaries are choking it off.

 

Go bigger. It's not just a number you're leaving on the table, it's area under the curve. Start around 5 mins:
I am presuming you are (re)tuning for it so there is literally no downside to bigger primaries - only upside. I can tell you from experience big primaries will help both on a stock LS1 and a cammed LS3.

 

I've seen the same.