Both my 2008 and 2006 Priuses (can't remember the words) drop out of EV mode at around 42km/h. Is it possible to increase this drop out speed to maybe 80km/h (50mph) My plug in extended range battery pack 2006 Prius will do 82km/h in the "Ice Kill" mode so the electric motor must be able to still drive at that speed. The problem with relying on the "Ice Kill" mode, it can't be switched back to mixed mode if more speed or acceleration is required or the plug in battery is discharged, without stopping, turning the vehicle off and then restarting. Not real convenient on a big hill with a B double boring down on you at 100km/h plus and there is no where to pull off the road. T1 Terry
Advanced topic that not too many know about anymore. @3prongpaul over at Boulder Hybrid Conversions in Colorado could tackle that,, but I have not seen him online for while. Back when there were many Prius hybrids but no plug-ins yet on the market, Colorado gave large state incentive for plug-in conversions. So Paul set up shop there for Prius conversions to plug-ins.
@Skibob is correct. In the Gen 2, Toyota set that limit for EV speed to protect MG1 from spinning too fast and exploding inside the transaxle. It's a gearing issue. In each generation since, they found ways to change the gearing and improve on the previous limit.
Short answer is no. However can go a little faster by getting to S4: If the Prius is in S4 warm up stage (reference https://techno-fandom.org/~hobbit/cars/five-stages.txt), it will allow the car to go up to 42mph (~66kmh) in electric mode with the ICE off. I recommend getting a scanguage if you don't have one already and monitor the coolant temperature. Once the coolant temp hits 73C, allow the car to idle with the engine on and the car fully stopped until the engine stops (takes anywhere from 5 to 30 seconds); the engine will give a bump or shutter just as it turns off, letting you know you've achieved S4. Also if the engine is not on, you need to turn it on by holding the brake firmly and press gas just briefly to start it, then wait for the engine to go through its calibrations for S4.