You're seriously comparing a BRZ to a Cayman....You can have a neutral FF car with 60F/40R weight distribution, and you can have a neutral RR car with 40F/60R weight distribution. Handling balance is not fixed by weight distribution... The ancient 911s that exhibited understeer until you gave a mid-corner lift at which point they oversteered off the road behaved that way because of suspension deficiencies and not due to weight distribution.
FWIW I drove my ~55/45 BRZ and my ~45/55 Cayman back to back at a few track events this year, and I can assure you that the Cayman both understeered less at corner entry and put its (modest) power down better at corner exit while being every bit as driftable as the BRZ. Both have minor suspension mods for front camber (-3.2 BRZ, -2.9 Cayman) and slightly stiffer and lower springs
50/50 is marketing for the masses who don't know better. It's also a mark that is achievable for a mass-market FR car, so when manufacturers hit it, they like brag about it, even to the point of calling it "perfect", which it isn't... For sure 50/50 for a rwd performance car is *way* better than 55F/45R. But 45F/55R is better still. For serious power/weight rwd cars, even more rear bias is better. Primarily better for putting power down accelerating out of corners (as well as in a straight line from lower speeds), but there are other benefits including better braking due to rear tires being able to contribute a lot more.
There's a reason megaperforming rwd cars are not 50/50...
There are soooooooo many variables there, I don't even know where to start. Chassis stiffness, quality of suspension, tire, front engine vs mid engine, overall build quality and chassis tuning. My God, you might as well compare a Chevy Volt to a Honda Civic Type R.
Anyway, back on topic, I don't think the balance between the I4 and I6 will matter much. IF there is any improvement at all, it'll be negated by the power difference.