Last fall I replaced a set of 205/55R16 H-rated tires (Goodyear Eagle RS-A) with T-rated tires (Michelin HydroEdge) on a '96 Cavalier Z24. Now I'm no Colin McRea by any means but I do tend to have the occasional aggressive streak and I didn't find my driving style was compromised in the least.
Doing average highway speeds of 120-130 km/h was perfectly fine. One of my rims was slightly bent and the RS-As (2 successive sets) always had a tendency to vibrate but the vibration actually disappeared with the HydroEdge tires, so they ended up feeling more stable at high speeds. The HydroEdge tires were infinitely better in the wet although the road noise factor due to the harder compound ended up being a bit more than I would have liked.
So, based on your father's driving style, I wouldn't worry about the speed rating drop, as long as you're dealing with a quality tire. That's my opinion.