i worked at a porsche dealership, trust me... they are the same
the cayman may not have quite as robust bearings but the block, heads, etc are all the same. there are different intake and exhaust manifolds due to the different car layout but that's about it on the physical parts. the 911's engine sits completely at the rear of the car with the flywheel facing forward going straight into the trans/diff. the boxter/cayman is identical, just flipped around with the engine in front of the rear axle line going into the trans/diff combo