The problem with a snaffle mouthpiece in a shanked bit is that it is two bitting theories fighting against each other. The snaffle mouthpiece works like scissors pinching the jaws when pulled straight back like you would in a shanked bit. With a ported shanked bit, it applied pressure to the roof of the mouth with the port(dependent on size and shape of port) and tightens the headstall over the poll...hence the importance of teaching a horse to give to pressure on the poll.
When you try to combine the two all you get is a really confused horse. I have a bit that has a snaffle mouth but with a large dog bone in the center, this seems to have a similar feel to a ported mouth because of the breaks in the mouth but not so much tongue relief. I still takes a certain horse that likes this bit and I have yet to find the common denominator, other than maybe being more tolerant. lol.
So I have skipped that bit and went to that short shanked square port with much better results.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.