When considering Mir + XMir or wayland + Xwayland, you have a full X server + the Mir/wayland layer compared to a full X server, so the performance can only be equal at best.
Regarding your Mir/wayland taunts, please consider that Mir is not (should not) be criticized on a technical basis, as it's architecture is very similar to wayland, although more targeted / less generic.
What is considered "bad" is that they could have entirely (and as easily) implemented the whole "Mir thing" as a wayland shell, fitting every technical necessity that they have put forward so far for Mir. Instead, they created their own API, which although being open source, is not made to be used by any other DE apart unity, so that their work will be absolutely useless for the entire gnu/linux community except for canonical.
And this as been (justifiably) considered as a dick move by said community, which has (understandably) stirred some harsh (and sometimes not so rational) comments.