I had my 1983 W123 converted to R134. It was properly done with R134-compatible compressor, expansion valve, dehydrator, hoses and o-rings. The molecular structure of R134 is smaller than R12 and it WILL leak through the hoses and o-rings if they are not replaced with the right ones.

Performance in Virginia on those days approaching 100 deg has been fine for the past 5 years since converted. I think with the addition of a good cross-flow condensor performance will meet or exceed that of an R12 system. If I lived in a hotter climate, it would probably be essential. I may look for one anyway.