There must be something, you would struggle to gain this increase in BHP from 163 to 184 just with engine management control, I would say a bigger Turbo but hopefull somebody else will know for sure.
My point for writting was to say DON'T do it, refuse and ask for the model you ordered and paid for, you don't want a brand new car with near 0 miles having engine parts removed, then I would assume engine management control programmes changed before you even get in it.
Your dealer is trying to pull a fast one, yes they can write it is a 184 on the registration documents but for ever and a day the chassis number is recorded on the Vauxhall systems as it was built, so before you actually sit in it the car will be modified from its original specification, every time you need an engine repair and a dealer types in your chassis number it will be looking for 163 parts or software....
You should stand up for whats right now, as soon as they have your money and you sign the documents it's all your problem from then on, politely decline the dealers kind offer ( which my suspicious mind might actually be a deliberate mistake to give you a stock car they have already) and ask for the car you ordered and have on your order sheet. A new untouched 184. If they get funny politely (keep the morel high ground) ask for your deposit back and walk away. I am confident you could find another Vauxhall dealer in the space of a half hour drive in most locations in the UK..