Warp ten is infinite speed. How can anything be faster than that?
No, it's not. Take a look at the chart in the ST:TNG Tech Manual. It shows that to go Warp Ten requires infinite power. It even says "Power usage approaches infinity." How they read their own chart to later say that Warp Ten is infinite speed is beyond me.
So what you're saying is warp ten = infinite speed.
Old sources get overwritten all the time. The tech manual chart said one thing, the show went on to say something else.
Anyway, back to the fan fic, he could do what TNG did and rejig the warp scale to allow more factors below infinite. "Go to Warp 15" does sound more dramatic than "Go to Warp 9.99932" or whatever.