Not likely to do any damage, but it is not going.to help either. If lack of lead is the reason it runs poorly, then the exhaust valves are already ruined and nothing short of a valve job is going to fix them.
Like Ken said, start with a major tuneup, including compression test & valve adjustment. If you find one or more exhaust valves that are tighter than spec, you might possibly benefit from lead substitute. But normally a TR6 should not need it.