As someone who runs mostly PCA track days I'll chime in, I also instruct so I've driven the cars on your list:
Boxster, the early cars had issues with intermediate shaft bearings. While not huge numbers of cars suffered from this, if it does go it trashes the engine (bearing bits in oil) and a used motor is 5-7K. The bearing kit is around $600 but the labor can be upwards of 3K.
911SC, 15K may get you a car but not likely it's going to come with complete service records, I'd say you would need to up the budget to around 20-25K. Also the A/C on earlier 911's isn't the best.
As for the S2000 and the lack of torque, maybe my time road racing piston port 2-strokes and inhaling bean oil fumes has altered my brain, S2000's have plenty of torque so it's not an issue. Yes you need to rev them a little more.............but that's part of the appeal, remember Soichiro is the guy who made 22,000 RPM 4 stokes successful nearly 50 years ago............it's part of Honda's heritage just as American V8's are know for grunt. Now I've only driven S2000's on track but found them to be a lot of fun, nicely balanced and decent power.
As for the M3, great cars but the insurance can be a real stinger and finding one that some zooty boy hasn't hammered on can be tough. Also note as they are somewhat heavy your tire bill is going to go up.
As for canyon carving, any car will do, I can drive along fast enough in my 140HP Protege to both get arrested and be glued to the back bumper of high end sports cars if I wanted to. A perfect example of this is take a look the lap times for front running Spec Miatas at Willow Springs...............they're equal to or better than many high end sports cars.
My vote is for the S2000, if not that get a Turbo/supercharged Miata or even a V8 Miata (if 15k will do it). I will also tell you that having a very fast single seater (SCCA D-Sports Racer) that faster cars do not equal more fun...... bigger rush but not more fun. The only thing faster than the car is the rate money flies out of your wallet. At most track days there are guys like myself in peddle cars; Miatas, Civics and old Datsuns going much faster than people with horsepower cars. It's that old adage of going fast in a slow car verusu being that guy with fast car and slow lap times.
Now do keep in mind I have my motorcycles so cars short of 600HP don't seem that fast, this also makes me lean toward an S2000 as the motors pull decent revs.
Tom