"Big valves are not absolutely necessary, mine ran for years on stock valve sizes , if you look at Mark Parkers port work, he gets much improved flow by tweaking the ports using standard size valves."
Yes if you have a flow bench etc for testing, you can get very good results with port work.
For an amateur a set of larger valves and a little work with rotary files/emery will get some
extra flow and a noticeable improvement on a road bike.
Also 90% of the blokes who successfully raced these things never saw a flow bench.