Or in other words, by 768kHz, BumpUp double can play C7 faster than Naive Int16.

The new discoDSP Corona R2 has my version of this. I think I got some interesting variations out of it, check the super organ* patches for example.

When you make the detuned frequencies multiples of the base note frequency, you maintain constant pitch range in terms of cents (or semitones). With linear detuning you get constant beat rates, but the higher notes will sound a lot less detuned. With exponential detuning (ie freq multiples) the beat rate varies, but the detuning in terms of pitch range (in terms of cents or semitones) stays constant whatever the frequency, so you can have half-semitone spread from the lowest bass frequencies all the way to Nyquist.

I've always wondered why everyone insists on linear detunes when the expo version sounds so much better to my ears unless one limits everything to a very small pitch range. Exponential is closer to how choirs, ensembles, even analog detuning tends to work. Things like chorus also gives you similar spreads.

As far as the exact ratios, it's not a huge deal what you use. A distribution where the "middle" frequencies are closer to each other and the "edge" frequencies spread more tends to maintain pitch perception better as there is more energy concentrated close to the nominal pitch. You might also want to sanity check that the beating ratios are sufficiently "random" that you don't get strong periodicity. I bet Roland's engineers just tried some arbitrary numbers and tweaked until it sounded/behaved nicely. Significantly different distributions DO give significantly different character, but ignoring the "badly behaving" coefficient set, slight variation isn't very noticeable most of the time.