I've read the 2nd pipe in a jdm exhaust is a resonator and a uk car is a second cat. Is this correct?

Also I'm not too bothered about how loud the exhaust is but would changing the resonator to a decat be less restrictive and make any gains?

Yes I would have to mapped in.

 

A resonator is not a restriction so removing it will only make it slightly louder and won’t require a map tweak 😉

 

I don't mind loud exhausts but if it's a resonator and twin scroll leave it alone cause it turns into a horribly annoying rasp.

 

The 2nd exhaust pipe on your JDM Hawkeye has a pre-tax price of ¥69200 ( about £485)
Not exactly cheap for a resonator.