It's a shame to hear that Yankee Dusty is rushing still and you've had some really good advice so far!
The thing is, like everyone said, a bit is a temporary fix.... you changed to the pelham from the full cheek snaffle, because you felt he was a bit strong and someone said above that once the power of the bit wears off it's gonna become a problem again. I really don't think the pelham is helping you at all... I once had a mate who rode her horse in the pelham... for the same reason... and this problem was the main one she had... she found that if she corrected it by stopping the running, she was able to switch back to an eggbutt snaffle and have no rushing and still the same amount of control.
I think you need to do LOTS of tempo playing... vary up the speed of the trot, like uhh i think it was tazmanian devil said... and remember to half halt! Loads of half halts are handy with a speeding SB... believe me, I know!
I too would cut the jumping out... if he's running off and you're struggling to pull him back and get him into a comfortable zone, I'd hate to think how hard he's gonna be whilst in the air! I know he's a good boy and that you're really enjoying the jumping with him... but I think it's still time to slow down...
My mate said to me once something she had learnt - A good walk leads to a good trot, and a good trot leads to a good canter. Yes he's being awesome and cantering! But maybe it's time that now he's got his basics, to bring it all back to the walk and supple him up. Most people forget how important the walk is... Like I've been telling ya what I've been doing with Evo, make a point of doing lots and lots of exercises to help him balance up and supple... do LOADS of walking... and then once you feel that he's going so much better, go into the trot and repeat the exercises. Make sure you aim for that nice, happy working trot... and not accept running off. If he does run off, bring him right back to whatever speed you were working at and carry on with it. He needs to learn that rushing is bad, and only going to unbalance things.
Good luck... I really like hearing about you both... it's nice to find another person out there with a SB who is experiencing some of the things I have or had previously, and being able to try and help.
♥ Seoul Searchin' for the Lovebug ♥