In my experience the Flex bit going on my 1.4 didn't affect performance, or MPG at all (all MPG is recorded lol).

But in general a blowing exhaust normally does.

Jon.

yeah cut the old flexi out mate and weld in a new one
cheapest option
make sure you weld it in correctly and that the joint between the centre section and the downpipe from the manifold is sealed
i replaced my centre section a couple of years ago with a spurious section and it never lined up correctly.
couple of weeks ago i cut the 2 flanges off and welded it up solid
no more blowouts lol