Black still has problems to solve in that variation.
Opening Discussion
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Well, maybe there is a better winning move? 
Anyways, nobody has looked at the variation so why not?
6...Bxf2+
This move is second best to 6...Nxb4. Black can acheive equality with it without very much risk.
7.Kxf2 h6 8.Bc4 hxg5 9.d3 g4 10.c3 Qe8 11.Ke1 Kd8 12.Qd2 Rh5
White has a small advantage, but black has acheived equality.
Summary
Three moves for black after 6.b4?!
A: 6...Nxb4!
This is the best move, white is now in trouble. This is the only way to challenge white's plan.
7.Bb3 Rf8 8.0-0 d6 9.Nf3 Bg4 10.d3 Qe8 11.Nbd2 Kd8 12.Rb1 Qg6 13.Kh1 Nh5 14.h3 Qh6 15.Nc4 Nf4 16.Bxf4 Rxf4 17.Ncd2 g5 and black has a significant advantage.
B: 6...Bxf2+
This is a second best move, but black acheives equality.
7.Kxf2 h6 8.Bc4 hxg5 9.d3 g4 10.c3 Qe8 11.Ke1 Kd8 12.Qd2 Rh5 =
C: 6...Bxb4?
This move is the worst of the three. White will have a good advantage after this.
7.Bd5! Rf8 8.0-0 d6 9.c3 Ba5 10.d4 exd4 11.cxd4 h6 12.Nf3 Nxd5 13.exd5 Nb8 14.Bd2 Bb6 15.a4 a5
5bxf7 is just horrible
Conquiscador wrote:
White has a small advantage, but black has acheived equality.
Bit of a contradictory statement there, wouldn't you say? 
cj_SKYHIGH wrote:
5bxf7 is just horrible
According to my database its not horrible.
OH RLY YYYYY
Is it necessary to run down the other lines having already refuted 6. b4? with 6... Nxb4, out of interest? Surely there is no need to have more than one winning move.