Of course you will get a MUCH better picture compared to S-Video. S-Video has all the color info mixed. RGB is every color seperate, exactly how the CRT "wants" it.
That is also why RGB signals have been around from the very start of color TV's, in the end fase, where the TV signal are completely decoded, there you have the RGB signals that were amplified and then run to the electronbuns on the CRT.
However, since there was no devices providing these kind of signals until much later, there was no connection made for it (in the US even untill today, regretfully for you guys).
The first SCART connectors appeared on TV-sets in 1977. SCART has been mandatory on every TV-set sold in France from january 1980. I think it's one of those times that government enforcement was a fantastic thing happening ! Since then, the SCART connector became the main connection on all TV sets, VCR's, DVD players etc in Europe and still is today. HDMI is just starting to take off now...
Anyway, let's have a picture of that connector. Remember you won't be able to directly feed it with a VGA signal unless you use software like Soft-15Khz !!
I've replaced the monitor in my Galaxian with a regular TV-set and connected it through SCART and I promise you that there is NO way you can tell the difference between an arcade monitor and this picture. After all, an arcade monitor is just a TV-set without a tuner. Since RGB bypasses the tuner completely, you have exactly the same.
In fact, using TV's is about the best option for replacing/using monitors on cabs here in Europe. First, real "arcade monitors" are hard to get AND very expensive here. Second, CRT TV's are VERY cheap (both new (€100) and used(€30)) and are plenty around since people are buying flat TV's like crazy.And ALL of them have SCART

I remember that Mitsubishi was a well known brand for multi-standard TV's back then, so it would figure that they included RGB...let's have that picture