Not a lot then but better than nothing. I've got a custom built stainless system from the cat back and have increased the fueling so may get a little more than a standard car. I'll give it a go and let you know.
p.s. how are you planning to get around the emission test on the MoT with no cat. I'm running mine on lpg for everyday use and only use petrol when I race it so I shouldnt have a problem