If your car is an early JDM then it NEVER had 300 bhp from the factory, it would be 276 bhp. So really you've only lost 10% which could be down to your exhaust and age.
I wouldn't even trust them with that!
Would recommend Martins in Winchester as an alternative... Recently had good sales and service experience there. Small dealership but part of a large group.
I LOVE YOUR SIG