Your customers will be bending the header to wrap around the cylinder to align the exhaust with the muffler. Once it is bent that much it's ability to flex is compromised. The bent part becomes much more rigid than the straight section and all vibrating forces will concentrate where the bend ends and crack apart. The stuff is called ridgid flex conduit. The "ridgid" part should indicate it's not supposed to be used in a moving/flexing environment. Now if you had designed it to have a flexible coupler on both ends of the bendable section there shouldn't be near as much chance for failure.
Has this been tested with many hours of actual flight time in various applications and with many kinks and bends as your customers will be expected to try?
Or are you just expecting your stooges to fob it off on an unsuspecting public as the next greatest thing since sliced bread?