I've just bought ROG Maximus XI Code Z390 LGA1151 and a 9900k, I already had 4 x 8gb Corsair Dominator 2666 so I used it.

I ran a benchmark before the upgrade at
https://www.userbenchmark.com/UserRun/17732954 and got faily decent results

After the upgrade I ran the bencmark again
https://www.userbenchmark.com/UserRun/17750900 but this time a got poor results

XMPII is enabled which seems to yeald the best results but I can help feel like I've missed something to get the score back up