I would never put bigger valves into any head that had nice stock seats, use those seats up by riding and not by hogging them out with misled dreams of extra easy power. Most every instance where I saw people put hot-rod parts and home porting etc. into a car or bike ended up making the car slower or no faster than it was.
One old friend of mine ported out his Trumpet head and the bike lost all sorts of low-end power, he ended up throwing the head out and buying a stocker to put back on it, and he got all the power back he lost with his home port job.
I'm quite pleased when my British bikes start, run reliably and get me home again.
The cheapest route to more (reliable) horsepower is to buy something from Japan