The cayman uses the same 74mm TB as the 3.6 996 and early 997 3.6/3.8. The gen 2 DFI 3.8 L engined 997 uses the larger 82mm TB. Regarding the mounting tees for the 74mm TB the cayman 987 and the 996 / 997 both use a larger diameter bore on the cross section that links the two halves of the intake manifold. Whichever you fit , you need to install a silicone reducer to get it to connect to the existing 986 / 987 intake manifolds. The 2006 and later boxster/ cayman use the 74mm TB and revised intake manifolds as standard in both 2.7 and 3.4 engines. I did a little write up on the 74mm TB fitted to an early 987 3.2S here :
75mm TB and Cayman Plenum upgrade for 987 - Articles
Note : If you fit a 987 airbox and MAF to a 986 (as was standard in the 550 anniversary) the install applies though the second diverter after the throttle body would not be connected or operational on a 986. The intake manifolds of the 986 were carried over to the early 987 models up to 2006 when the M97 engine was introduced.
For you guys with 986's I guess you have the option to break out the inner wall that divides the air paths post TB and remove the diverter flap as well. The cayman diverter flap looks wrong to me , its across the airstream creating a baffle whereas the earlier 987 boxster unit had the diverter parallel to the air stream , so created no restriction when open.