It is heavier at at 3367 pounds although Subaru should be commended for producing an all wheel drive turbocharged four-door in the 3300 pound range. It has a smaller 2.5 liter boxer four-cylinder although it is turbocharged. The STI has more power though rated at 305 horsepower and much more torque with 290 lb-ft to the Cayman's 213 lb-ft.
The STI gets the lead off the line thanks to all wheel drive traction and holds it through the 1/4 mile. The STI runs 13.8 @ 101.9 to the Cayman 14.2 @ 101.2. The Cayman is starting to reel the STI in ever so slowly past the 1/4 mile marker though. The elapsed time at the mile marker is even between the two cars with a 35.8 second sprint. The Cayman is pulling harder at the mile marker though at 140.4 miles per hour to the STI's 135.2.
So what does this tell us? That if you own an STI race a Cayman from a stop. If you have a Cayman, race an STI from a roll. The longer the race the more it favors the Porsche. Additionally, it would be interesting to see the difference the PDK transmission would make here as it should make the 1/4 mile much closer between the two.
Overall a very well done video with supplemental data from Automobile.