If 15.Bxd8 (instead Bxe4) then
19.g4 Bxb2...this is black is better with 1 pawn up.
Compared this with 17...Rxd8?!
18.Bd3 then black still a pawn down.
After 17...Nxc4!, its not wise for white to force the event for being greedy.
18.Nxa7? Rxa7 19.Bd3 Nxb2 20.Bb5...this position black is MUCH better white h-rook just caged and white a-rook obliged to guard his a-pawn. The 2 black rooks and bishop are very active and white d-pawn is also will fall...0-1.
18.Bxg6!? is better
21.bxc4 Rc1...this position also black is better white h-rook still caged although black is a pawn down, it would be soon paid off because the activity of his 3 pieces, example:
27.Kh2 Rxc4...black got a pawn back and still better with superb activity.
22.Rb1?? (instead Rc1) is bad
23.Nxb1 Re2! and now either the a-pawn or f-pawn would fall with the threat of Bd4
So the conclusion is 13...b5 is a good move and honesty, its a complex one, you need at least oozing a sweat to think the variation of the line.
Beside 13...b5! also there is 13...a6 ...its a quite move and promising a solid game.
But for me 13...b5! is my choice