Here are some quick facts about the discussed cars:
-Due to smaller engines in the 911 and Boxster/Cayman, getting more HP from simple bolt on mods are much harder than getting power from an M3
-Cayman is a very capable car from factory. The mid engine platform gives the Cayman a lot of traction and easiness to drive for any driver with any experience
-The new Cayman S will also feature a 3.4L Flat 6 as the Boxster S, and the base 991. The engine is a de-tuned base 991 engine. Meaning you can get more power with a simple tune.
-Porsche will always keep the Cayman the little brother to the 911. Otherwise it will kill the 911 sales
-The driving technique for 911 and M3 are completely different due to platforms (FR vs. RR)
-The M3 has advantage over the 911 in the weight bias. The M3's good weight bias will help going into the turns. The narrow tires of the M3 and having a lighter rear end will result in oversteer coming out of turns
-M3 has its disadvantage because of the hefty weight of the car
-M3s are certainly under-rubbered in the rear, but not on the front
-The 911 is light on the nose, meaning it will induce understeer going into turns. The technique to drive a 911 is this: slow in, fast out. The heavy rear weight will yield to better traction coming out of corners. So an M3 driver may keep up going into a turn, but will fall behind coming out.
-Mastering the skills to drive a 911 to its limits will certainly take longer than an M3