We're still on 1.7.x and using TSM (not TSI), because we've found that to work well enough for the time being. Haven't tried 2.x yet because the investment needed to properly benchmark and upgrade would probably be around a week at our scale, and there are just more pressing things to do.
To be fair, we still haven't paid you guys a dime so I can't complain. But if you're interested in hearing my thoughts I can email you after next week's holiday.
I haven't looked but the latest 1.7.x is probably close to 2 years old. There have been quite a few improvements since 1.7 so you might see some improvements by upgrading to the latest 1.x.
Minor nit on "TSM (not TSI)" - TSM (.tsm files) are the data files and that format hasn't changed. TSI is the newer, although mature at this point, indexing option that spills onto disk and can therefore be larger than the original in-mem index. You're probably using in-mem indexing.
We're always interested in feedback. My name is david. You can email me at <name> at influxdata dot com.
To be fair, we still haven't paid you guys a dime so I can't complain. But if you're interested in hearing my thoughts I can email you after next week's holiday.