I'm not going to pretend like I could tell you the solution to the oil control problem your vehicle seems to be suffering from standing here; yes, seals could be a participant, but his actually often is only in conjunction with excessive valve guide clearance too. But so could the piston rings be a cause, as remember rings utilize cylinder pressure to aid in their seating and on a long downhill run with the throttle closed your not making a lot of pressure to aid the rings in seating or participating in oil displacement through "blow-down" either.
Also, the fact that umbrella seals may become "hard" doesn't really change their effectiveness, they're actually not well defined as a seal at all, rather they would be better described as an umbrella "shield" or "oil-deflector"; that is until they crumble and fall off!
B.T.W. the Comp-Cams #514-16 will not properly fit "stock" heads, as they are intended to press over a .500" guide boss O.D. (these are not an "umbrella" seal) and this the "stock" heads do not present, unless they have been machined at a later date for such.
Scott.