However, I found out that without grounding, just the leakage from parasitic capacitance in the power supply was enough so that the chassis would float at 1/2 of the line voltage, 60V here, and with enough current to give you a palpable shock.
No,it wasn't "parasitic" capacitance it was the "Y" capacitors on the interference suppression module.
The 300 mA RCD is there to trip on earth faults in equipment, such as burn-out of a metal-jacketed element where the element shorts to the jacket.
The 30 mA rating is specified to protect against people actually connecting themselves from line to ground. This is a historically later consideration in the regulations.