I think the performance gains are quite substantial and other than the years where they've added extra cores, one of the bigger jumps from year to year. You've around 30% faster single core, 15% on multicore and graphics anywhere from 50 - 75%. This is excluding the other benefits of the higher end model like more ports, extra fan and better speakers.
If you look at the 10th gen H series chips (Ones in the MacBook Pro 16) then they are around 7% faster than the 9th gen and have no faster integrated graphics at all. That's not a big gain.
Having said that it still doesn't mean you'll notice the difference in day to day usage. It depends what your doing but if your just doing some office docs, browsing and so on I highly doubt you'd be able to tell the difference. Computers have long since been fast enough for those kind of tasks.
Even on my wife's 2013 MacBook Pro 15 it's still more than fast enough on those tasks. It'll browse smooth as butter in Safari, Office apps work without any issues and even though the SSD is limited to SATA speeds so it's around 4x slower, apps still open pretty damn quickly. I mean I expect when my MacBook Pro 13 arrives I'll notice it opens apps much faster but it's going to be like 1 sec vs 3 maybe. 3 seconds is still not that long is my point.
So nothing to feel bad about if you go for the 8th gen, it's still going to be very fast and capable. You might not even notice the difference to the 10th gen outside of benchmarks but saying the 10th gen performance is disappointing I think is also misselling it.