First thing's first—yes, you should take this potential update with a grain of salt, because like any anonymously sourced report it could turn out to be bogus. But Best Car Web has been a consistent source of reports on the Supra GRMN, and other calls it's made like the next-generation Toyota Land Cruiser dropping the V8 have turned out to be true in our own reporting. ?
The S58 can currently be found in BMW's X3M and X4M where it produces between 473 and 503 horsepower, however the engine is expected to get a bump in power when installed in the Competition Package spec of the German brand's classic M3 and M4 cars when they launch later this year. At 510 horsepower, this version of the S58 should theoretically be the most powerful. It's also the one reportedly going in the new Supra GRMN..
It's especially unusual Toyota didn't go with a stick considering a.) the M3 and M4 will offer one and b.) supposedly it's only going to make 200 of these hotter Supras.