maybe scanlines help but I think SNES games look better with the 2xSAI engine. I also use tv out.
And I always wanted a 2xSAI engine for NES emulators but...
And I was really just talking about fill rate; I don't know exactly how the graphics core would handle a string of NULL flags during rendering... but it's probably done using a post filter of some kind. But oh well, what do I know, I know very little outside of BASIC...
I think if scanlines are programmed well or whatever they can actualy improve performance after all they do blank out half the screen. I remember nfs on my old 133 having scanlines on gave me around 50 fps compared to the 20 i'd normaly get