I have a slightly more mathematical take on things, perhaps. So, why do I think that extratone should be 1,200bpm and higher? We shall see.
First is that the low bound of human perception of frequencies is 20Hz, which is 1,200bpm here. Extratone at around 1,000bpm does not feel so much like extratone, and its defining feature,
being too fast for human perception that it is read as a frequency instead, often a low one, and 20Hz seems reasonable here, for such reasons as frequency boundaries.
My second reason here, is mainly an aesthetic decision. Speedcore usually is considered to start around 300bpm, and go to 600bpm, which is twice 300bpm, and further if splittercore is to
start at 600bpm, and go to 1200bpm, that is twice 600bpm, which is a second iteration in a pattern. Human perception as well is pretty logarithmic or exponential in nature anyway, as the
difference between 5 and 10 is generally considered much larger than the difference between 305, and 310, and larger that than 600,005 vs 600,010.
My third is related to the first, but still distinct, somewhat of an analysis and appendix to it. It is this, 1,200bpm being the low for extratone lines up well with 1,200,000bpm being the
low for hypertone, as 1.2Mbpm it is the higher bound for human frequency perception, at 20kHz, it is as well, 1,000 times 1,200bpm, which is fun. To have them both be boundaries of human
frequency perception is very cool.
As well, such a thing is largely arbitrary anyway, Einrich3600 early on wanted extratone to be 3,600bpm, which I disagree with, but I see his logic. Plus, 3,600bpm is 3 times 1,200bpm, so
that may be some credit for you.
I understand reasonably well for it being 1,000 bpm as it is, that it's the first number with four digits, and so on like that, and it is decently fast. However, I have tried a more math
based approach.
This analysis does conveniently leave out supertone, which I think is mostly fine, anyway.