After you left I had to look up the SIO technical specs, and you were right about the Happy speeds being faster than 19.2k. It's spec'd as running at 52k. The SIO has a theoretical speed of 120k, but you have to remove two capacitors and add a resistor in order to unlock that speed. It was originally capped at 19.2k because of two things: 1) the FCC putting a limit of how much interference was allowed and 2) Atari at the time didn't have any equipment that could handle speeds faster than 19.2k and therefore weren't able to measure interference above it. But the coolest thing was the 6532 MOS chip in the drive was called the RIOT (Ram In/Out Timer)...
I was confused as well. It seems that the limit is set in the OS in combination with POKEY and with a modified ROM and driver you can get 120k/bps. The 4.7k resistor and removing capacitors was to improve the reliability of high speed transfers. Stock components can reliably achieve 58k/bps, although the 1050 drive with the RIOT chip was limited to 19.2k. The Happy and other drive mods used a 6502 to replace the 6507 which was able to utilize the higher transfer rate. I gleamed a bit of this information from the Fujinet project and a Github repo HiassofT/highspeed-sio...
