They may be better than stock. However, I had the Kooks installed and I have not had any problems with the gaskets. Some of the fasteners did loosen up once and had an exhaust leak and I think one of the fasteners worked all the way out and was missing. But I took the car in and had everything tightened up and drove it another 500 miles or so and had them double check everything was tight. No problems since and it has been 2 or 3 years since then.
Make sure you use the proper fasteners as my Kooks came with 2 different set of fasteners with different threads.
2013 GT, Track Pack
, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets
, JLT Oil Separator(s), Barton 2 Post Shifter Bracket
, Steeda Motor Mounts
, Barton Shifter
, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft
, BMR Loop
, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts