I think there are 2 main reasons for bashing on Mac headers:
1) Individual flanges on the header primary tubes - Flanges may warp from heat over time and cause problems lining up the bolt holes and may cause exhaust leakes
2) HTS coating - This is High Temp Silicone. It is a protective coating that is supposed to protect form corrosion, but is scrapes off easily. I would rather have this than bare steel though.
I think these are valid concerns, but not enough to label them crap. There are A LOT of people that run Mac exhaust. If it sucked that bad, they'd be out of business. There are several local guys that run Mac and have no problems with them, even after a head swap they still line up fine. I am getting Mac LT's soon too. There is a lot of heresay, but I am going off of what I have seen here with my own eyes and that is several people with no problems at all. I say go for it! I would bet they are higher quality than the Flowtech's that you have now.
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1991 Mustang GT - Cobra Intake, FMS 65mm TB, C&L 73mm MAF, GT-40P heads, Crane 2031 cam, Crane 1.7rr, Built AOD, Art Carr 2400 non-lockup converter, Transgo shift kit, BBK chrome cold air, FMS 'P' headers, Pacesetter off-road H, BBK aluminum pulleys, 2-C Flows w/ dumps, 4.10's, Steeda STB, for now...
The Original GT-40P Tech Page