My integrated tail light (meaning it has the turn signals in it) came with a resistor for this very issue and it did, indeed, flash really fast. I didn't bother putting in the resistor and when I got around to putting in semi-flush mount front signals, they must have had internal resistors since the rear now flashes that the OEM rate.
So there's the chance that if you do an aftermarket tail light that has an internal resistor that your entire system would revert to the stock flash frequency. YMMV.
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