The E60 headers are the MAJOR restriction AND the biggest power to gain if done correctly. To get to Z06/997TT level all you really need is a de-catted header and exhaust. If you buy catted headers, you might as well keep the stock ones and send me the $4000 to my personal bank account, is the same result, money lost.
Headers are not a bad mod, actually there the best mod to these cars, of course done the right way.
I'll let you know...I bought Agency headers and will have them on my car next week sometime with a pulley and an ECU tune. Apparently they will still pass emissions, yet are catless...
I've heard the headers make a big difference, esp. with an ECU tune. I think Agency headers are the most reasonably priced at $3k. Everyone else is crazy expensive. Jerry at Eurocharged stands by them, so I took the plunge as well and I'll let you know next week what I think!