Old japcrap cars gain 25bhp by opening up the exhausts as they are known to be very restrictive.
Modern day performance turbo cars dont see so much as they are a lot more efficient and opening up the exhaust too much may cause issues with not enough back pressure being produced. I had a 3'' Dp and sports cat fitted last week, although its helped top end, it seems to be more laggy down the bottom end.
Full exhaust system on our cars, probably 10-15bhp max. Your better spending your money on a remap where in the LCR's case, you can see improvements of 50-60bhp/100lbft