to angle mill will ... change valve to piston angle . change the location of the valves relative to the bore center , unshrouding them. it changes cc's which will increase compression and more. all are tools which can be used in conceptual forms. most [true performance] modern heads of today have taken many of the "ideal changes" that angle milling achieved and designed them in to the production version. there is a lot to be gained by doing it if you are stuck with iron heads from a rule standpoint or other reason in my opinion. it can be expensive to do it correctly by correcting head bolt hole faces and intake angles etc.....I have even moved the dowel location to get the valves better centered !