Some input:
Consider a decoherence process modelled by $$\mathcal{E}\left(\rho\right)=p_0\rho+(1-p_0)\sigma_z\rho\sigma_z$$ with $p_0=(1+e^{-\gamma t})/2$. One can readily find that this leads to $\mathcal{E}\left(|i\rangle\langle i|\right)=|i\rangle\langle i|$ for $i=0,1$ and $\mathcal{E}\left(|i\rangle\langle j|\right)=e^{-\gamma t}|i\rangle\langle j|$ for $i\neq j;i,j=0,1$. With the linearity of the quantum operation $\mathcal{E}$, properties of tensor products and $\mathcal{E}^{\otimes n}(\sigma^{\otimes n})=[\mathcal{E}(\sigma)]^{\otimes n}$ one readily finds that the effect of $\mathcal{E}^{\otimes n}$ on a GHZ state $$|GHZ\rangle=\frac{1}{\sqrt{2}}\left(|0\rangle^{\otimes n}+|1\rangle^{\otimes n}\right)$$ can be evaulated by only looking at the "off diagonal terms" in the density matrix (regarding the computational basis $|0\rangle,|1\rangle$) $$\rho=\frac{1}{2}\left(|0\rangle\langle 0|^{\otimes n}+|1\rangle\langle 0|^{\otimes n}+|0\rangle\langle 1|^{\otimes n}+|1\rangle\langle 1|^{\otimes n}\right),$$ since the others don't change.
We have $\mathcal{E}^{\otimes n}\left(|0\rangle\langle 1|^{\otimes n}\right)=\left[e^{-\gamma t}|0\rangle\langle 1|\right]^{\otimes n}=e^{-n\gamma t}|0\rangle\langle 1|$. The exponential factor is considered as the decoherence rate of the ghz state, since it determines how quickly the state becomes maximally mixed (in the computational basis $|0\rangle,|1\rangle$ I assume). So far so good.
Now consider the generalized ghz state $$|GHZ\rangle=\frac{1}{\sqrt{K}}\left(|0\rangle^{\otimes n}+|\epsilon\rangle^{\otimes n}\right)$$ with $|\epsilon\rangle=\cos\epsilon|0\rangle+\sin\epsilon|1\rangle$ and some $K$ to normalize the state.
Question:
Is it true, that one can (here) also consider only the "off diagonal terms" $|0\rangle\langle\epsilon|$ to compute the decoherence rate?
My thoughts:
No, because here we cannot use that $|\epsilon\rangle\langle\epsilon|$ or $|\epsilon\rangle\langle\epsilon|^{\otimes n}$ is unchanged, since $\mathcal{E}(|\epsilon\rangle\langle\epsilon|)\neq |\epsilon\rangle\langle\epsilon|$! We would have to consider all parts of the density matrix and write it respective to the basis $|0\rangle,|\epsilon\rangle$ (or $|0\rangle,|1\rangle$ ??) to compare.