Compiled with golang 1.6 & 1.5 zurichess is about the same strength, but with golang at tip (which has a modern SSA backend) zurichess is about 30 ELO stronger in self-play. Given that huge ELO gains from search & eval are rarer and rarer I welcome this boost .
i too have seen improvements over the last couple of months. a lot of work went into the compiler/runtime recently (e.g. ssa rewrite, as you noted.). this big jumps speedwise is an indication that there is still a lot of room for improvement. exciting times ahead for go developers!