pH is one of those things that really skews a lot of perceptions. The pH of my tap water is 8.8 - 8.9. Conditioning for 24 hours brings it down to 7.5 - 7.6. It's lower, but still very high according to discus literature. Yet, my fish thrive because I make very large water changes very often. In the summer (the outdoor temperature currently is 12F), I do not condition water so the change water is sitting at pH 8.8 - 8.9. Again, the discus thrives because 1) the water is clean and 2) water parameters do not change. I raise fry and adults in this water and the adults will spawn successfully under these conditions.
I expect that your fish will have similar, if not more, success along as water hygiene is maintained, particularly because Stendkers are spawned and raised in high pH and KH in Germany.