I've been to a couple Porsche events since getting my C2S and have witnessed (among other things) several GT3's from 996 to the current 991. People drive these cars hard and put them away wet, and yes, they were quite a bit faster than the gaggle of FR-S/BRZ's that were also present. But yes, they cost a lot more to run, and pray you don't crash one or blow the engine. It really all comes down to budget. If I could afford a GT3 I'd get one, but I can't so I got a little ol' Carrera S... which I pray I don't crash or blow the engine
An FR-S/BRZ could definitely hang with a GT3 if it were set up right, but it would definitely cost some $$ and a lot of time to dial it all in. The GT3 already has it mostly figured out for the driver; just get in and go.
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Current: 2005 Porsche 911 Carrera S 6MT
Previous: 2 BRZ's, 997 C2S, C5 RS6, C4 S6, B8 S4, GDB STi, S30 240Z, FC3S RX-7 TII, AW11/SW20 MR2, E30 318is/325i, etc.