Basically anything changed after factory is a modification, anything non-standard and anything changed "after-market" - even if it's a factory option but added on later, if it wasn't on there to start with then the car has been modified from it's original state and therefore different to it's leaving the factory spec.
If no change in power then you shouldn't see much (if any) change on your policy if you're with a specialist insurer (especially if you're on a modified policy anyway), if you're with a standard or budget insurer they may charge. It really depends on who you're with and what type of policy you're on.
Carlie