how important is it to increase valve size?

i have called around for some prices and get any where from 400 to 1000+
and although i find it hard to believe the are all telling me it will not create more hp or be beneficial to the head
but i have done a lump port install on it and the paper that came from hotrod6 said bigger valves were of benefit


Josh
72 gmc lwb air ride 5 speed (soon) turbo 292 II# 6102