Yeah, I can get the Neo Geo working in 3x mode, but I just felt like using the tweaks suggested in the op may be messing with the ratio somewhat? I could be wrong, it just looks like a different ratio to the the 4:3 mode my TV produces in x2 mode and my Mega drive using x3 using the tweaks in my post above.
I am not sure which is ‘correct’ mind, as I don’t have access to a CRT to compare! And I presume this kind of thing can vary from console to console? There’s certainly a lot of overscan area on an AES!