X15 is not better than X11. Both of them are as secure (or as insecure) as the first of the algorithms used (Blake). Possibly as insecure as the weakest of the 11 algos (i'm still trying to prove that formally, but my math is falling short, so i asked for a friend who is an experienced cryptographer for help). If that is really the case, then X15 is even weaker, since it has 15 points of failure instead of just 11.

Only advantage of X15 is that there is no FPGA for it yet (but, as for X11, it is not hard to build one, since the algorithms used are all NIST competitors and all were required to have published FPGA implementations). But then, the signature part of spreadX11 does the trick better than adding 4 extra algos. (and makes the algorithm stronger, since now to break spread you need to break sha2, ECDSA and blake, not only blake)