Once you enabled trim, the drive is now better capable to self manage storage without needing any special help.
You could kick start the process by making a backup of the drive (just in case the filesystem has corruption) any then boot into Recovery HD and use Disk Utility to repair the drive once (which is functionally equivalent to huy's answer). That intentionally trims all unused blocks when the filesystem is otherwise idle.
I would avoid any attempt to overwrite free space since that just adds more needless writes to the drive. If you feel that's needed, a total erase and reinstall from backup might faster in the end.
You are correct that the first immediate delete operations could leave you with free blocks scattered about non-continuously, but writing a gigabyte or two temporary file and deleting it should get enough free trimmed blocks to make an immediate speed up and the rest of the speed will come after garbage collection cleans up after routine file changes and deletes pass through trim status.