I used them at first because of the initial cost of the headers and exhaust work. They lasted about 90 days, and had to do them again. Went to a local Mustang shop and talked to him and all he used for his builds were Cometic Copper ML gaskets. The deal was cheaper than Motorcraft. I bought them and have not had a exhaust leak since they went in. That was over 30k miles ago.
I'll be using a different kind with the turbos, but Ill be relying on a complete sealed exhaust system them. That $50 gasket might save me 5000 bucks in the long run. But I dont see you having any issues if you use the copper coat. I used it on my headgaskets too.
I'll say again the Ford gasket are the best but are $$$! I'd be willing to best that the cheap OBX LTs are the issue with the leaking not the gaskets.
There is a very reputable Ford performance shop that's local to me. I contacted him about tuning a customers truck for me after installing OBX LTs(that's what the customer wanted) and the owner of the shop highly recommended not going with them. Said he has installed about half dozen OBX kits and all but one leaked. He now refused to install them.
He wasn't trying to push any other brand either, actually suggested in not even install LTs.
But that's just my 2 penny's. I been around the L/HD world a LONG time and have seen many issues with using OBX LTs and or none Ford gaskets.
Silver 2013 F-150 Supercrew 5.0L FX2
Boss Coilover Suspension-Nitto Terra Grapplers-Airaid Jr Intake-Banks Monster Muffler-MagHytec Rearend Cover, AMP Steps-SCT Tuned By Torrie @ Unleashed Tuning
I would buy a set of header flanges from Jegs and weld both flanges(the obx headers and the new flange) together. Shouldnt have any problems with leaks with the Jegs/Kooks flanges bolting up with the felpro gaskets.
I never had a issue when I had my OBX's. But that was of course before they went to the cheap 1/4" flanges. I now have a set of Pacsetters, and despite the stupid slip fit collector I have had 0 zero issues with leaking. But of course Pacesetter is a slightly better made product in my opinion. I like my nice thick 3/8" flanges. the seal great with copper gaskets.
An before @Z7What comes in I only used copper because I live near the beach and the headers were ceramic coated and the coater didnt complete clean the mating surface and I did have time to have them reground flat.