I'd second that.
A few years back I had a (non-fault) accident in a Clio, when the estimator came out, the damage was the last thing he looked at.
First thing he did was look under the car, at the brakes, suspension etc, then in the engine bay, and finally at the stereo - all to see if there were any undeclared modifications that could mean they wouldn't have to pay up.
Then he turned his attention to the accident damage.
Like has been said above, a strut brace is bloody obvious, and could easily cause additional damage to the opposite inner wing / suspension turret in a crash - if that happened and was spotted by the insurers, you're going to be out of pocket big time.
And whether accessories are made by Seat or not, if they weren't fitted in the factory as standard then you should declare them as modifications.