Well, I guess this is the $64,000 question.
It can be hard to quantify "good improvement" because one person's subtle improvement is the next person's huge difference. Hi-Fi marketing (headphones or otherwise) often treads on the minutiae and tries to convince people to feel bad about their existing product with hopes they will abandon it in favor of the new model.
From my (incredibly biased) perspective, the new ER4SR's primary improvements are in the quality of the build and materials. The actual frequency response does get a little bit closer to our target curve (which makes us happy) so in that regards it's better, but it's a slight evolution, not a completely different sound. I'm going to leave it up to the listeners to quantify the differences. Whenever you change something that's been around for 20+ years, there will always be some that prefer the new model and some that prefer the old model. If I had the choice, I'd buy the new model over the old one, but again, I'm incredibly biased. For those who love their existing ER4s, I expect you'll really like the new one, but I also expect it will not make you want to throw your old one in a wood chipper.
The ER4XR is a bit different. I never really considered the ER4P to have a bass boost. Yes, it was a bit warmer sounding because the frequency response tilts with the lower impedance. This has the result of reducing the high frequencies as well as increasing the lower frequencies a bit. But it's not really a pronounced rise in the low end; it's different. The XR is basically flat with a tuned bass lift. While it's not a 15dB thumping boost, it does have a different sound than we've offered before. Of the two models, this is the biggest departure from the old sound. This is trying to address the requests over the years from loyal Etymotic fans that said that they like the sound quality but want just a little bit more low end.