Hey, I would suggest DINAN. I work for a DINAN dealer so I know the products. I recently installed a HiFlow T body, air box and mufflers on a 4.6is. Good difference and great sound from the mufflers. You would need programming if you do the airbox and t body though. The mufflers don't require programming.
REMUS has some X5 modifications. Their exhaust is widely known and i believe they have different stages of modifications. They supercharge i believe, and recently they do 4.6is mods. For more information go to http://bimmer.roadfly.org/bmw/forums/e53/ and ask around.
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~ZyajZarc|craZjayZ~
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cheap. reliable, or high performance -- pick 2, that's all you get.
have you tried looking at performance speclists like HARTGE, HAMMAN, AC-SCHNiTZER ,RMS and maybe autowerke ,Racing Dynamics, MK Motorsport , RMS and of course DINAN.