So apart from frequency range, what am I getting more than the 8010 and less than the 8030?
Level handling. Peak 4 dB more than the former, 4 dB less than the latter. As you go from the 5" class to 4" and 3", bass handling worsens substantially at each step.
Effectively, if you still want 103 dB SPL peak, maximum listening distance should be about 1.3 m for 8030, 0.8 m for 8020 and 0.45 m for 8010. I would recommend somewhat less. (The 8040 is the first model that could be used >3 m out, reaching beyond direct sound dominance even in a well-treated room.)
https://www.genelec.com/correct-monitors
Now while hiss levels would generally be going down for the smaller models, self-noise spec actually is the same for all 3, so let's assume it's not overly different. This may eventually leave you in a pinch with inadequate dynamic range. So as the difference in level handling is larger than the difference in price, I would recommend not going too small either.