For 240p content, linedouble mode’s horizontal scaling is currently limited to 1x (i.e., the equivalent of “Generic 4:3” setting for line3x-5x). It would be great to have the low-active-pixel optimized modes (320×240 and 256×240) available for line2x as well, to enable 2x horizontal (nearest-neighbor) scaling with linedouble.
One specific reason would be to facilitate N64 “de-blur” for line2x in the same manner you can currently do it for 3x and above (i.e. with a H.samplerate of 387). More generally, it seems most 240p consoles with 320 or lower horizontal res show additional noise and/or blur when oversampled by a factor of 2 (even when it’s exactly twice the native sample rate).