I love the myler comfort snaffles. Of all types. I am no bit guru but heres my two cents.
For western pleasure, since your horse is older than 5, I believe you will be required to ride a shanked bit. They arent all the devil. If he goes light in a snaffle, he will go light in a shanked bit. The second bit (the shanked one) is basically a tom thumb without all the confusing mixed signals. I think it would probably be a very good transitioning bit.
As far as speed events, that kinda depends on the training you've put in. To me, a good gaming horse could be ridden on an o-ring snaffle. Its more about reinforcing the responsiveness in more than just w/t/c. That needs to come from leg, seat aaaand hands. Hands being last. I think a bit like that has more than enough brakes for even a hard runner.