Not a sports car, but I was driving a not-very-stock 2006 STI since 2006 before getting my M3LR this year. The STI has been modified over the years and is without question a fast and nimble car on the track, or anywhere. I've not dyno'd it in a while, but several years ago I went on a "HP" quest/obsession and upgraded the turbo, radiator, intercooler, exhaust, injectors, and ECU (and tons of non-engine stuff too). With a conservative tune it was pushing past 450HP at the axles. I could have squeezed out more, but this was also my daily driver and I didn't want it exploding.
The M3LR is such a different car. The acceleration, even without the $2k upgrade, is incredible by comparison. The STI was no slouch, but getting it to do its theoretical maximum acceleration of about 4.2 seconds took a lot of work and practice, and despite my best efforts, suffered from turbo lag and terrible shifting (my fault). So even if the STI and M3LR have identical 0-60 times, it feels much faster in the M3. The STI feels like a wrestling match. The M3 feels effortless. The STI going that fast is partly luck. The M3 going that fast is a sure thing.
I'm guessing the STI might beat it on a smaller track with few straights, but it'd be close. I'm sure the STI weighing 3,400lbs doesn't hurt. But the STI is loud, exhausting to drive, and always feels like it's ready to catch fire. An hour of driving in the STI feels like running a 5k.
The M3LR is comfortable, smooth, and can warm my butt while it beats the STI's 0-60 and quarter mile times easily. It's also quiet, which makes it easier to ask my passengers if they're enjoying the acceleration.
I sort of hate the M3LR for making me realize that the STI I'd put so much effort into isn't that appealing to drive now. Nor is the idea of filling its 10g tank so it can gulp fuel at 12mpg.