If I recall correctly, the endlinks on the STi have to bend around either an axle or a control arm in a C-shape so they have to be strong to endure those forces. The Boxster's endlinks are straight, and they have a ball and socket on each end. Therefore unless the endlinks managed to bind up, there could not be any torsional loads on them, only axial loads. There would be no reason to make the endlinks fatter - they're plenty thick for the loads that they see.
You'd never see any performance benefit from going to a fatter endlink. But to clarify, the M030 endlinks for the US and RoW are the same as those used on the non-M030 cars.
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