I'm currently using the Peter Sullivan mouthpiece that came with my yamaha 882,it's ok,but my chops get really tired playing lead comin to the latter stages of a gig.Just wondering if anyone had any suggestions?

I don't think it's a question of right mouthpiece for the horn. In your case, if you're doing a lot of lead playing, it sounds like you're using the wrong horn for the job. I really don't think going to a smaller mouthpiece is going to help you all that much in the fatigue department.
Any chance you could use a smaller horn? Something more suited to lead playing?

Thanks Steve for the advice.The only thing is that smaller bore horns just don't do my sound any justice.I played the King3b for 5 years and never felt comfortable where as with the yamaha i feel right at home.I just seem to have a hell of a lot of control over my tone and quality of pitch with the yamaha.What horn are you using and what's your main work?

I use both. For lead playing, I use a 3B, and for more classical or wind ensemble stuff, I use a large tenor. The two horns don't sound the same, and I wouldn't want them to. It did take me a while to embrace that small horn sound. But now, I approach them almost as two entirely different instruments.
There is a big tendency to overblow a small horn if you're used to a big one. It takes some getting used to.
Right now, I'm in our brass quintet, so I'm using a large tenor a lot more these days. But for the last two years I had done primarily lead playing in our big band and our Showband. For that I used either my 3B or a Bach 16 .
It all comes down to using the right tool for the job.

Ah,a well rounded player Steve!!I get what you're saying about the right tool for the job,I'm just stubborn and probably will return to using the 3b when i get sick of fatigue in the latter stages of a gig.I would just love to use the yamaha,such a beautiful tone from it.Never had the pleasure of playing bone in a quintet,but did play tuba for 3 years in one until I realised that it just won't pay the bills!!Now I'm just a musical prostitute with damn all morals left!!

Don't just do it for the endurance issues. Really consider the blending issues and projection. Lead playing in a rock or big band setting require a much tighter, focused sound that blends well with the lead trumpet player behind you. Big band charts tend to require attacks and articulations that just tend to sound tubby and muddy on a large horn.
I realize there are a few guys out there that can pull it off, but unless you're Robin Eubanks or the like, you're most likely better off on small equipment in that setting.

i used a gold plated bach mouthpiece once, and i may say, it just feels comfortable