I was thinking because of the difference in engines the quote would be higherWhy not?
If you were to do the same thing with your 328i, it would always be officially known as a 328i. Furthermore, there would be no obligation to divulge your modifications typically. If you wanted to specifically insure the modifications, you may need to divulge them, but chances are an insurance company will still only give you insurance for a 328i, regardless of what you may have done to it. Other than CA, the state won't care.