I don't have a direct comparison to a G8x M3/M4 but I got the wife an LCI X3M comp that I drive alot as well.
I previously posted my thoughts between the two.
Here it is copy and pasted:
The Blackwing wasn't a car that instantly blew me away like the X3M comp did. I was immediately impressed with what a feral beast that SUV could be despite its body shape and weight. I had the X3M Comp for my wife for about 2 months before the Blackwing arrived so it was an immediate point of comparison. All the reviews talk about what a beast the CT5 blackwing was. I was naturally surprised by how civil and refined the Blackwing felt, to the point of being subdued.
Going hard on a backroad, both vehicles will thrill and excite. There is so much power and grip in both vehicles. The X3M would probably be faster in my hands on a back road given the AWD putting down the power well, the amazingly fast ZF 8 speed and the superior visibility. A good driver would definitely be faster in the Blackwing, but there is definitely a greater sense of fear and danger there too. Both cars are talented opera singers on the open road, stirring the right emotions and captivating the soul. The Blackwing hits higher notes here and there but the X3M is close enough that it's shocking.
The difference is when you come up on a car doing sensible speeds on that back road. The BMW falls silent, frustrated that the fun is over. The Blackwing seems to do the same at first. Listen closely however and one realizes that she is still there, singing that same wonderful tune as before. Now, instead of belting it out at the top of her lungs, she's whispering it sweetly into your ear. The steering still communicates with light tugs in your hand. The body motions are still perfected controlled. The exhaust still rumbles with perfectly rev matched down shifts (by the car not me) like distant thunder. It will do this even at 1500 RPMs coming off the throttle. How aggressive you are the revs merely determines how far that tempest sounds, but it is always there. At legal speeds the Blackwing shows off its greatest talent, it's ability to entertain at 2/10ths. Driving the Blackwing requires so much more of my attention. Occasionally I drop of my wife at work and use her car to make the commute to even out the miles between the cars. I'm always flabbergasted about all the extra roadside stuff I notice in the X3 that I just never paid attention to while driving the Blackwing. Was that church always there? I don't ever remember seeing this overlook. It's not just the higher vantage point, because it's all stuff visible from any ride height.
At the end of the day, the X3M feels Iike a vehicle that a large team of people allocated some of their time and resources to, while the Blackwing feels like a vehicle that a small team of people devoted all of their effort and passion to. When asked to perform, both the X3M and Blackwing will sing beautifully. However when doing the banal every day chores, the X3M becomes a perfect domestic, and you nearly forget that it's anything other than another X3. The Blackwing can nearly those same chores, but she is always humming that melody while she is working. Even when doing the most boring of tasks, she never ever lets you forget that she was, is, and always will be, an absolute superstar.