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There have been significant scalability improvements in that time (9.2, 9.5). There also have been lots of smaller improvements in single threaded performance in general, leading to rather noticeable aggregate improvements.

That's all for the cases where the same query plan is used in 9.0 and 9.5 - the differences where new type of plans (say index only scans) are often very large.

In short: yes.

Off the top of my head, Index only scans, overhauled GIN & GiST indices, Trigrams come to mind.

9.5 was supposed to get "parallel sequential scan" (which is a big deal performance-wise), but I don't see it on the "what's new in pg 9.5" page. Anyone know what's up with that?

It wasn't ready yet. A significant amount of development happened after the point where that's supposed to happen for features to be integrated in 9.5.

Now that 9.5 has its own branch I hope we'll get it in early in the 9.6 development cycle; so we have time to iron out all the potential kinks.

I don't think there ever was a clear "this will be in 9.5" statement from anyone?

9.5 was the target according to this: http://rhaas.blogspot.com/2015/03/parallel-sequential-scan-f...

Thanks for the info.