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Back in 2017 I had my car modified at Abbey, I had an ACE CS400 manifold installed, alongside a Cobra HFC. Performance is great, torque dip eliminated. However I was never that happy with the sound. It was okay-ish with the rest of the exhaust stock but I kept looking for options to make it sound better. Last year during lockdown I purchased a Q300 catback, which is meant to be one of the quieter systems, but I still found it a bit loud and droney at about 2.5K RPM. I got rid after three months. I then went to just a Remus backbox, still with the ACE and the Cobra HFC. Much more bearable than the Q300. But was still getting a little bit of annoying rasp sometimes at low revs. So last month, I decided to see if removing the HFC made a difference. I swapped it out and went back to the OEM front pipe, which has a resonator. That has got rid of the rasp, and I haven't really noticed any loss of performance or efficiency. That said, I'm not a track driver, and probably don't drive as hard as most, so I might have lost a tad of top end power but not noticed. But I am at least happy with the sound now. So the exhaust system now goes ACE manifold > OEM front pipe > OEM midpipe > Remus backbox. All that is a long way of saying that in my experience, I've found that the system sounds better with the OEM front pipe, and I've haven't really noticed if going back to it reduced power. If you're getting a decat manifold, just be aware that a high flow cat might make the system noisy if you have anything other than an OEM catback. Whether that bothers you or not is up to you. As for MOT compliance....I think the general consensus is that with a decatted manifold, and the second cat in the front pipe replaced with a HFC, you might be at risk of not passing emissions - at the very least you will have to make sure it's tested with the exhaust piping hot to make sure it passes. Mine has never failed the MOT since it's been running with just the Cobra HFC as the only cat...however that's because I've made sure to always take it back to Abbey for the MOT, who will make sure to test it when it's thoroughly warmed up.