Not to mention the 45 series changing your handling.
Usually when you go wider, you go lower aspect ratio to remain at the same circumference. The engineers set that car up as a package with thousands of hours of development time. Unless you're looking to make specific changes, I'd suggest you stick with the stock sizing, as @eliongater suggests.
Here's a visual comparison of your stock size, and your tyre professional's suggested option:
Aside from the handling change (taller sidewalls adding 33mm to the height, more sidewall flex, and higher c of g), you can see it'll also gear it up, and nearly 99mm to the circumference (more than 5%).
Seriously, I'd shop somewhere else, where the tyre guy has a clue.