CAI and Exhaust are not proven to give more power. If you did the full exhaust: a freer flowing cat + x-pipe + exhaust it's more likely, but no one has posted dynos to prove it. Headers will still provide more power than full exhaust.
Proven ways to increase speed: euro headers, tune, more aggressive differential gearing. The other thing you can do is lose weight: racing seats, lightweight battery, lightweight wheels, etc.
Losing weight is often overlooked, but if you look at the 2012 cayman R (which has a 4.2 0-60) it has virtually the same HP/torque figures as the Z4 M but weighs 100+ lbs less. There are a LOT of other differences (PDK, etc.) but you get the idea.
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