I would get them changed if you can. Although the changes are small, none will make your car illegal (the difference between 16" wheel tyre combo & 19" wheel tyre combo is significantly more than what you are looking at) 225 section tyres are fitted to smaller diameter wheel cars so that shouldn't be a problem. What would be a problem is if (and only if) the insurance companys cry foul if you were involved in a serious accident and there was an in depth investigation and they were not aware, as alfa do not specify this size tyre on this chassis (technically it is now a modified car).
Bearing in mind it's going to cost about a £1000 to change them to offer to change them free of charge, they must think its pretty important - and I can't see them getting that money back on yours.
You don't have to change them, but if you don't I'd make a make a phone call to check your insurance company are aware of the situation - just to be sure.
Insurance is a contract of utmost good faith and you are obliged to inform them. Some aren't bothered, some may treat your car as a modified car & up your premium, but you must let them know.
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