I²C FM+ (1 MHz) on STM32
which could go up to 3.4 MHz (e.g. Kinetis K20 chips).


That's likely hip specific. I have never used any i2c devices that far but 500 to 600kbos is not uncommon, though technically not supported.

Pushing it over 1mhz could be tough. If I were to Doo it, I would make sure that more drive current. Is available (use lower value pull ups on the bus), watch out for stray capacitance / inductance (short and shielded wiring), and most important of them all, make sure that the devices support that kind of speed.