I have talked with several old timers on this subject, and last season Mike strausberg was at our track, his mechanic builds my friends Engines and I asked them this question. They build all race motors, NITRO, or race fuel withOUT EX valve stem seals. They said " The Ex valves do alot of work and take alot of heat, so not running the seals gives a little extra room for oil on the stems for lubrication and cooling, they can get more life out of them this way. I ran seals on them last year but when I re built my heads for this season I did not install them. I hope this helps and makes since.
I also questioned my machinist and he also agreed as long as its not a street motor dont run the seals on the Ex valves.