But those are lines most people want to lose. There's a reason the AVS cuts them off by default.
5x height with 6x width output only cuts off 5% on the top and 5% on the bottom, both of which are in the overscan area of most CRTs. It also achieves a PAR of 1.2, which looks pretty much perfect. The pixels are razor sharp too because they map 1:1 with what gets displayed.
Outputting 720p is more cost effective and it's probably fine for most people. But 5x by 6x output is better if you care about integer scaled with accurate PAR.
1536x1200 image rendered to a 1920x1080 display only causing a loss of 120/5 = 24 actual pixels. 12 pixels on top, 12 on bottom. 5% top/5%bottom or a total of 10% of your vertical picture.
Just so happens that the NES/SNES actually draws to 224 and has an overscan of 16 pixels already so really you lose even less.
I would imagine this is what you are talking about right?
What is the resolution when rendering 256x240 to 720p with a proper stretch of horizontal? Would that be a 4x horizontal, 3x vertical to 1024x720 on 1280x720 space? Hence the full vertical space being used?
4:3 720p being 1.333333 PAR
6:5 1080p being 1.2 PAR
Actual target would be 8:7 or 1.14 PAR from what I can gather. Requiring a 4k set. 2048x1680 on a 4096x2160 display with significant black borders. Only other 4k integer scale that makes sense would be back to 4:3 which is pretty far off from 1.14 relatively speaking. Big advantage on 4k being shaders doing an aperture grill or shadow mask I guess.
Setting up Retroarch for that 1536x1200 image seems to make sense for Nintendo but I think for genesis/mega drive you are going to want to go for 1:1 PAR as it is supposed to be 0.914 and I don't think you get close with integers on a 1080p set going with any other scaling.
For the purpose of the AVS what are the scaling factors done on the horizontal axis on the vertical integer scaled 720p image using that stretch feature?