Board Thread:Questions and Answers/@comment-25908228-20141222101442/@comment-27384850-20161218052856

1) Kawahara said that the use of Excalibur would give Kirito a possible win due to the stat difference, not due to Kirito's skill in any way.

2) Kirito, in a Q&A, can be quoted on his knowledge that he is only at 80% of his peak (that being the end of SAO) and can also be quoted that he believes that Yuuki would win even if he were at his peak.

Yuuki would still beat Kirito more often than not, even with Excalibur he's said to only probably be able to win.