This core has apparently a constant aspect correction built in, choosing between 1:1 par and core provided ratio makes no difference. However, the aspect correction seems to be also applied in a wrong way.
Taking a non-gpu screenshot of a game (in my example, Winter Games) results in a 320x223 screenshot.
The core turns this into 320x240, i.e. it stretches it vertically. Applying the TV-Out Interlacing shader results in this:
Correcting via custom scaling to 960x669:
This probably isn’t the accurate ratio, but at least provides proper scanlines.