By the way, video signal on cps2 arcade machine was indeed 16:9, but the rendering was on a 4:3 screen. Rendering cps2 games as 16:9 is a bug, it was fixed on lr-fbalpha in late 2015, and was backported to lr-fbalpha2012 a few months ago.
About using the old lr-fbalpha2012 because you got the wrong roms, why not update your roms instead ? If you buy the wrong tire for your car, do you buy a new car or do you go back to get the right tire ? I can understand using lr-fbalpha2012 on a raspberry in a few specific cases (i'm mostly thinking about svg and killbld here), but lr-fbalpha is overall better because a lot of bugfix and compatibilty was added in the last 5 years.