Haven't used the syringes but the regular Nupafeed made a positive difference to our difficult Connemara - it meant he could be shod without ACP, and was generally easier to travel, etc - but he still bucked!
Also used the Carl Hester liquid calmer (Mag-Pro?) on a mare who is fizzy to ride but easy to handle, and I'd recommend it - again, haven't tried the syringes.