Is de-cat exhaust a modification?

[P6 TUS]

for insurance purposes?

[GreenV8S]

yes

[P6 TUS]

will it be costly or will they say it should not have been done?

[GreenV8S]

Obviously you haven't just done it without telling them as that would probably void your insurance, so give them a call and ask them what would be the impact if you decided to go ahead with it. I wouldn't expect it to make much difference, but then I'm not an insurance broker.