Is there a comparable difference between the retro race specced xc bikes and the current race spec xc bikes?
Not really. My mid/late 90s Ti XC HTs are both very similar to ride to my "still available in the shops" Carbon XC HT which is what i went to when the 90's got retired.
Top tube is a bit longer, stem is a bit shorter, but then, a zaskar TT was ~15mm longer than my Ti, so no surprise there, and there is an extra 30 odd mm of travel up front. There is also half a degree on the head angle, but the suspension actually works on the new bike, and it sits further into the travel, so its not that noticeable (nice active air/oil forks are so much better than elastomer!)
Thing is, back in the 90s, almost all HTs were fairly racy, there wasn't this "do it all" hardcore HT bracket, people just put short stems and wide bars on their XC HT frame. And tried to break them. Probably explains the rarity of the Zaskar!