=> [0 = -x^5+15*x^4-90*x^3+270*x^2-406*x+249]
This is incorrect. It is specifying the solutions of the
outer composed polynomial (polydecomp), not the
composition as a whole -- that said, I don't think there
is any notation in Maxima for specifying the composition
of implicit solutions.
Maxima 5.9.0 gcl 2.5.0