Mongoose branded exhausts used to be made at blueflame in Skelmersdale. That changed a while ago.

I'd go for a Blueflame if I were you. Stop worrying over 1mm

Thats 0.5mm at the top and 0.5mm at the bottom

The different design of the box and the internals used will make it flow better than standard so what your worrying about is nothing.