The Goodyear F1 GS-D3 does not come in the correct rear size (265/40/18), but the Goodyear F1 Asymmetric does, and it beat the PS2 (and others) in TR's comparison test.

N-spec tires sometimes cost less; sometimes more. TR lists three different 235/40/18 PS2 versions; the non-N spec version is $270, the N2 version is $267, and the N3 version is $265. Prices flip in the PS2 265/40/18 size, however; $326 for the N3 version and $315 for the BMW spec version.

I've run more non-spec tires on my Porsches than N spec tires, but I'm convinced that there are Porsche-mandated differences between the N-spec versions and the non-N spec versions. Michelin got it's N-spec PS2 version for the CS up to specs early, but BS and Conti were slow.