Don't know how to do it using tf, but you can do
syms s
Suu = -1.6/((s-4)*(s+4))
Sux = -0.8/((s+4)*(s-4)*(s^2 + 0.1*s + 1))
Sxx = 0.3*(s - 4.163)*(s + 4.163)/((s+4)*(s-4)*(s^2 - 0.1*s + 1)*(s^2 + 0.1*s + 1))
Sxu = subs(Sux, s, -s)
SxdSdx = Sxx - (Sxu*Sux)/Suu
factor(SxdSdx)
or, to obtain a factorization of the numerator also,
feval(symengine, 'factor', SxdSdx, 'R_')