¿Have Mamiya lenses improved since I used them in the 1970s?
Yes, very considerably.
I have noticed a strong correlation between lens design generation and performance, especially among the Japanese MF manufacturers. Mamiya, Bronica, Pentax and Fuji really upped their lens game from about the mid-1980s onwards. I find that those who are dismissive of say, M645 or RB67 or Pentax 67 glass, are typically talking about their experiences with some of the 1970s designs - like Dick here.
But all of the new Mamiya designs from the past 25 years or so have been excellent: all of the Mamiya 6 & Mamiya 7 lenses, the K-L RB67 lenses, the ULD wideangles and APO teles for the RB/RZ, the "A" line for the M645 (120 macro, 150/2.8, 200/2.8, 300/2.8 and 500/4.5), the ULD/APO 645AF teles (210/4, 300/4.5), the "D" 645AF lenses (45/2.8, 80/2.8, 150/2.8, 75-150/4.5). [The 28/4.5 D AF might be an exception though]. And it's important to note that the less appreciated AF lenses (like some of the wideangles) are actually old optics in a new AF casing - they're not really new designs at all, so they don't counter my argument.
That is not to say that some of the older "C" designs are not also stellar: the Mamiya 24mm ULD fisheye has never been surpassed in MF, to take one example, and used samples still command a 4-digit dollar price to this day, because it's just so damn good even on the high MP digital backs.
Ray