Olly, in the world of Porsche unfortunately there are limited options and none are cheap!
You can dial in a little camber on the standard top mounts, the mounting holes in the strut top are slotted, if your lucky you may get to neg 1.5 dependent on ride height etc - The down side being that by moving the strut top inboard to increase camber you lose track width.
Adjustable, two piece, lower control arms are the way the factory adjust camber, these are fitted to GT/Cup cars and available from Porsche.
Adjustable top mounts with the sliding adjustment are not commonplace, GT3 monoball top mounts have a motorsport mounting position which allows the damper to be mounted as outboard as possible to increase trackwidth while providing the maximum camber.
If your looking for circa neg 2-2.5, I would go for Porsche/Adjustable LCA's. - These with around 7mm of shim coupled with the slots in the strut mount should get you where you want to be. - From memory a pair from Porsche were around £1k, I believe the RSS items to be about the same.