Could differences in fuel line size cause fluctuations in fuel flow (but notl enough to not indicate pressure fluctuations)?

I have the stock fuel line to the filter and then I have -8 AN from the filter to the inlet of the TB.

I think any flow fluctuations would smooth out by the time they reach the filter. That's about 4 feet isn't it. Couldn't be any worse than elbows in the line.

I'd say you're biggest concern are the internal passages on the TBI itself. -6 line for supply and return should be plenty so long as your pump is healthy.