I currently own a BRZ. I bought it as a 'just in case' car, in case the new Z never appeared. I shopped long and hard for a new 370Z, my first choice. But the combination of my picky and specific requirements (red, Sport Tech, 6 speed) and most Nissan dealers I dealt with were complete sleaze, ended up in my not finding the car I wanted. We've had good experiences with a Crosstrek and WRX, so I bought a BRZ as a second choice. While it superficially seems comparable to a Z, it's a very different car, similar size but lighter, and of course much less power. It's undeniably fun to drive, but I like the 'old school' torquey V6 of the 370Z.
There is a LOT of rumor-mongering and speculation on the BRZ/86 forum about what the engine will be in the new GR86. Obviously, everyone is hoping for a turbo 260-ish hp engine, as that would address the biggest shortcoming of the current car - not enough power (though truth be told, it's enough to be fun in street driving). I personally doubt it will be turbocharged, and suspect it will be a naturally aspirated 2.4 with 217(ish) hp.
Toyota calls the shots in the car's development. While technically it's a 'collaboration', Toyota is about 10 times bigger than Subaru (in terms of revenues and market capitalization). I don't know the relative financial contribution of each company, but Toyota is clearly the 'senior partner'. I don't think they will do anything that would risk jeopardizing Supra sales. Especially now that the Supra is offered with a 4 cylinder 255 hp engine (though only with an automatic), a 260 hp GR86 with superior performance (and a likely $10k cheaper price) would cannibalize Supra sales. The sports car market just isn't big enough for both cars (maybe not even one of them). Even though Toyota buys the Supra from BMW, it's still their legendary brandname, Supra, the stuff of legends (and Fast and Furious movies). I doubt they would undercut Supra sales with the GR86.
The N/A 2.4 engine with 217 hp would have a modest torque bump from the current engine, but still wouldn't be anywhere near the same league as a Z engine. But, if Toyota/Subaru ever did see their way clear to allowing a 2.4 turbo engine in the car, then it would indeed be potentially tasty competition for the Z.
But when you dance with an 800 lb gorilla, you dance to the tune the gorilla calls, and you're finished dancing when the gorilla says you are. Toyota will decide what happens.
