If you are willing to look there are a few good changes in trunk for InnoDB. A frequent source of mutex contention in InnoDB, kernel_mutex, has been replaced with a rw-lock for the transaction system (see trx_sys_struct), two mutexes in srv_sys_struct and possibly other mutexes and rw-locks. The dulint struct has been replaced with a native 8-byte int. All of these changes should make InnoDB more efficient for workloads with a large number of concurrent transactions and help with bug 49169.
I wish there were a better to track changes in InnoDB. Right now my tools are luck and recursive diff.