Allocs of size 16 can bypass atomic set of the allocated bit, while allocs of size 8 can not.
Allocs with and w/o type info hit different paths inside of malloc.
Current results on linux/amd64:
BenchmarkMalloc8 50000000 43.6 ns/op
BenchmarkMalloc16 50000000 46.7 ns/op
BenchmarkMallocTypeInfo8 50000000 61.3 ns/op
BenchmarkMallocTypeInfo16 50000000 63.5 ns/op
R=golang-dev, remyoudompheng, minux.ma, bradfitz, iant
CC=golang-dev
https://golang.org/cl/9090045
