Not sure if you'll be able to get your mid-range back. The growler is a very open exhaust, and although good for top-end, eliminates most back-pressure, which is where your midrange torque comes from. Are you planning on fixing the bike yourself, or would you be opposed to a shop doing the work?
This is interesting... I think the UK has FI, no? I think FI should be able to handle a new exhaust no problem...
Who put on the exhaust? You? A friend? A shop? Does the exhaust have an O2 sensor on it? It would be located where the two headers come together - you can see this by looking the front of the bike, behind the front wheel, and following the two large pipes leading underneath the motorcycle...
Also, was this a full exhaust, or just a slip-on? Sorry for all the question, but the info is kind of necessary to diagnose the problem.
Met, go be a hero!