Possibly lean, yes, not sure if it depends on other elements of the exhaust system being changed or if that's enough to push it over the edge on its own.
I chatted to Paul at System-R about the Spoon 2-1, he did make a comment about them being difficult to get off for the MOT (not as difficult as a one-piece manifold like the Toda though!) but more importantly, there are a couple of bolts that do not like being repeatedly done/undone and can break as a result. Can't remember which ones though, sorry!
I think others on here have taken this route as a compromise on the de-cat/manifold upgrade vs. MOT situation, no doubt they can offer some direct experience.