Aftermarket Exhaust

I find the S1 too quiet, can't be bothered with hassle of a res delete. Will the car need a remap after the fitting of an after market cat back exhaust? Iv read that it is fitted after the catalytic converter so it shouldn't negatively affect the engine itself (for warranty purposes). But aren't they supposed to increase bhp and torque a little and this affects the engine from stock, and also how do I know how much increase there is to tell insurance because I don't need to give them a reason not to pay up?

I’d imagine you’ll get very little increase just from a cat back.

De cat or sports cat would see a healthy increase but you might need a map to get the full benefit, especially if removing any o2 sensors as you’ll get eml.