This is certainly possible. The only reason we haven’t changed the firmware to do so is for backward compatibility — we only started including microSD card from OS 2.3. For previous versions, one would need to install a microSD card to use the firmware. We don’t want to make this a required step.
We’ve done a survey earlier on the forum about compressing the water times, and I think the decision was unanimous that granularity of minutes is sufficient. Intuitively, the longer the water time, the less accuracy it needs.
You can feel free to modify the firmware to support higher precision. The compression is transparent to the UI — the firmware always decompresses the value when sending the data to UI, so the compression remains local to the firmware and the UI is not aware of it.