Yes, Segwit will effectively allow larger blocks, but that's not its main purpose, and the increase is marginal.
The most recent transaction type data shows that we would have a probable block size of 2.1 MB (up from previous 1.7 MB calculation). So 2.1x is what we call marginal today?
Well, I have missed that part. Last time I read about it, it was 1.7 to 1.8 MB, which is still substantial
Regular Bitcoin transactions will only be able to make use of 0.7 - 0.8 MB of the increase, because the 3 MB in the increase is for signatures only. Lightning will add enough extra signature MB's to use what standard transactions can't.
but still modest considering that some 18 months ago, there was a proposal from leading BTC developers to raise the block size to 8 MB.
When you say "leading BTC developers", you're referring to Gavin Andresen & Mike Hearn? I think you mean "former BTC developers"