Rheinberger: Mass in Es, Op. 109, 'Cantus Missae’ - Groot Omroepkoor - Live concert HD 

Does it matter? I have absolute pitch, but I just turn it off and listen to the chords' functions instead of paying attention to individual notes.

It really matters. I also have absolute pitch, but I cannot turn it off, especially when I hear choral music.

notaire2 I've spent a long time playing harpsichord, meddling with electric keyboards' tunings, singing in Baroque pitch etc. to "desensitize" my AP, and focus my brain on chord tuning. Transformation isn't all perfect but definitely fruitful.

I am envious of your brain's versatility, but it is not my taste to desensitize my absolute pitch, even when I sing in baroque pitch.

I like the work a lot, esp. the Kyrie, but yes, you're right, it is probably the piece which has challenged our choir most in the 25 years I have been with them - we dropped over a third, I think in the Credo, in one performance with a guest choir - a shock for all, not least for the conductors! It has long piano sections, and long phrases, so needs top breath control. There is no accompaniment, no retuning after some of the sections, and romantic key flexibility, so no well-beaten track. It also in some way is more sensitive to the building than most. This performance has a lot to commend it, though, and is worthy of the choir's top reputation.