Seems to me a thickness of 20mm or even 25mm on the front won't be a problem. I run 20mm on the front with a couple of different 9" wide wheels (212s, 87s (9.5 wide) and a set of a/m 20" Rials) each with slightly different offsets. Tires between 255 and 275 wide.

On the back I use 20mm or 30mm depending which wheels I am running. One example, 10.5" 87s with et30 I ran 20mm hub size adapters, giving effectively et10. That was pretty much as far outboard as I wanted to go (315 tires). A set of E71 212, 9" et18 with 20mm was almost equally aggressive looking (255 tires). Right now it is on the Rials with 30mm spacers and 275/45 winter tires (I think wheel is et40 or et45) and it looks quite nice, close to flush but not excessive. Since my last alignment I run low camber and that pushes the top of the wheel a bit outward as well.

Given that your wheels are 55mm (Porsche uses higher pos. offsets than BMW), 30mm is a good option. Going with 40 or 45, effectively et15 or et10 may possibly exceed the flares depending on tire size as well, but is still doable. Really depends a bit on what you can personally tolerate for a regular passenger vehicle.

Very curious what type of wheels those are, post a picture? Also post a pic of what you wind up doing and how it looks on the car? It is always of benefit for others as it can be a bit hard to know exactly what a wheel & spacer combo comes out like.

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2003 4.4i black ext./black int./black headliner (kid's runabout)
2014 535d X-drive, M package, silver/black interior

Sold but not forgotten:
2009 E70 35d, black ext./black int./black headliner (sold 2021)
2006 4.8iS Le Mans blue/cream int./black headliner, SOLD in 2012 sadly...

Other hardware:
2015 Cayenne S (wife's new DD and definitely wroooommmmm)
2016 Tundra 1794 edition crew cab, luxo hauler
2005 Tundra crew cab, weekend hauler. Sold after 150k very happy miles.