I'm not sure, if I really understand your question. If you mean rebuilding Gtk/Qt4 against the old version of libx11 but with xcb: no I didn't try it, because it seemed too radical and I didn't expect the gtk build to be different if it is built against a libx11 version with xcb and thought that xcb was disabled intentionally because of some bug. Furthermore there are some applications using some other toolkits like tk, that we'd have to rebuild, too.

TLDR: you might try it (especially if you have to rebuild the core anyway,i.e. thumb repo or those who work on the software for the neo900), but don't expect too much

@marmistrz: Merging without regressions (and without using all workarounds that were necessary for 5.1 but are obsolete or even bad for 5.2) is actually what takes the most time (at least human time - computing time is not the issue here).