The Triumph head was originally developed for a 790cc engine and it is now a 995 one with the same stroke. The intake valve centers are too close together and the exhausts, too, for this size bore. Up to 6mm larger valves can be used. Shrouding becomes an issue with valves this size unless a lot of metal is removed all around them. Moderate size valves are needed so they will be unshrouded without adding a lot of combustion chamber volume. Straightening the inlet tract, like Jim says, helps to avoid sonic choke problems. It means that smaller, better breathing, valves can be used.