did anyone here actually measure round trip or just trust the number that is shown in the DAW (or Reaper specially).

Most (if not all) USB devices don't report the internal USB buffer the device has.
Focusrite 2i2 in the driver you can set the latency as low as it can go on 44.1khz (the driver says 1ms, which means it's _adding_ 1ms).
Then Reaper shows +/- 9.3ms. But if you actually measure round-trip it comes out at 12ms, and that's the lowest it can go.

If you then switch the device to 96khz, the (hidden) internal USB buffer stays the same in samples, but since the samples are now shorter, latency is shorter.

Reaper still shows 9.3ms, but actually measuring round-trip now gets to 6.9ms. Your system needs to keep up with low-latency 96khz though :S.

My Saffire Pro 14 is at 44.1khz 128 mostly, Reaper reports it as 5.0ms (you can configure it's internal firewire buffer in the Focusrite software this time) which when measuring seems to hold up give or take 0.1ms.