The point of a big valve is not that you can run a bigger valve, it's so that you can run a bigger seat. If you put a bigger valve in a stock seat, think about what you have done. The air still only has the same area to flow between the seat and the valve stem only now it has a bigger valve "head" to avoid as it's flowing.
If you can get a bigger seat, do it. Otherwise, run the SMALLEST valve that will work with that seat.