First off, very sorry for the slow response, some web server settings were changed and forum notifications stopped being delivered. It’s fixed now and responses should be faster.
To answer your post requires digging in a little about how the Mooshimeter does its sampling. Because BLE has very low bandwidth, the Mooshimeter builds up a sample buffer of up to 256 samples, runs analysis on them (mean, RMS), then delivers only the result of its analysis over the BLE link to keep the amount of data transferred low. So that internal buffer can be sampled at up to 8kHz, which means events at that frequency are reflected in its RMS analysis, but the rate at which it delivers notifications to the next level is lower than that.
Why does this affect SD card logging? The SD card logging feature was designed to capture whatever would be sent over the BLE link, so it stores the calculated data as well. Because it was designed with the idea that someone would be putting the meter somewhere and leaving it for a long time, the way it’s written it assumes the logging will be slow, so after every sample it writes the data out to the SD card (which takes a few ms). So your description of maximum 300Hz sampling sounds about right.
There’s a “buffer mode” in the app that lets you look download the raw sample buffers and examine the data at its full bandwidth, but only in bursts, so it’s not exactly what you’re looking for. Unfortunately there’s no way to log the raw sample buffers to SD right now. I’ll put it on the wishlist though, I can see it being useful in other circumstances.
I’m really sorry the meter doesn’t support your use case right now, please let me know how else I can help. Best