I own the IE300 and a/b’ed it with the IE200 at CanJam, though that’s not the best listening environment so take this with a grain of salt.
Going in, I expected the IE200 to just be a retuned IE300 and since I EQ the IE300 anyway, I was sure I would buy the IE200 and sell the IE300 for convenience. Unfortunately, while the IE200 did have a more neutral tuning, I personally thought that the IE300 was still better in every other way, including the timbre that I love it for. With EQ, I think the IE300 sounds noticeably better than the IE200 ever could.
Honestly, I’m surprised Sennheiser discontinued the IE300 in favor of the IE200, especially since there’s still people for whom the IE300’s more v-shaped sound would appeal.
The IE600 I tried, on the other hand, was a noticeable and clear upgrade over the IE300, but I’m not really interested in spending that much on an IEM.