[Drifter]

Historically Toyota ECUs were nearly impossible to crack which meant aftermarket tuners could not rewrite fuel & ignition maps without changing to a standalone. From a cursory search, it looks like today some Toyota ECUs have been cracked, but I'm not seeing the Prius listed anywhere.

The problem with $150-$500 piggyback tuners is that they're almost universally against the law (they affect emissions in violation of the Clean Air Act). The EPA fined Bullydog $300,000 and forced them to perform millions of dollars in remediation. So there is a pretty big disincentive for capable people to make such tuners.

Of course, there are millions of people who want more horsepower and hundreds of thousands of them are willing to pay a fair bit for small gains so there is still a big carrot motivating tuners. And they can somewhat plausibly claim that their tunes are meant for off-highway use on race tracks.

There aren't a whole lot of people willing to pay a lot more for small mpg gains and tuners can't exactly claim that highway fuel economy tunes are meant for off-highway use.

Prius chat has a few DIY circuit modifications to fool the ecu into thinking it has warmed up sooner, but I don't know of any that attempt lean-burn. If I were going to attempt it myself, trying to shift the O2 sensor's output to a richer reading would probably be my first attempt. But I'd want to monitor everything else to make sure it isn't causing problems and I'd want it to turn off automatically at moderate and high loads.