I have never messed with 55 degree seats on any head that did not have titanium valves and beryllium seats. I know they flow more at higher rpm and less at lower rpm. If you are not going to spin it over 7,000 rpm, they are not worth the cost. But the higher degree valve angle is harder on seats, and the light weight titanium valves help the seats survive.
http://www.dragzine.com/features/valve-seats-will-50-55-degree-angles-work-engine/