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And your mp's are?

since you're already playing on a GW, why don't you get in contact with them and try their whole line to hopefully find a match? I know they have like a 14 day trial period on their mouthpieces and certainly have sizes that run the gamut form the toilet bowls (I have a Caver now, but am going to buy a Bayamo when i get the $ ) right up to tiny pea shooters (the Churada has no place anywhere near a CC/BBb tuba!). or, if you're near a well stocked instrument dealer spend a day and find what works best.

the unfortunate thing about the mouthpiece question is that its all about opinion. there are some generalities, "deeper = this" or "Bowl = that" but and the end of the day its complete personal preference.

In my opinion, the PRIMARY effect is the *volume* of the mouthpiece. Choose a rim size to match your face - and then choose a deep/shallow cup to get the volume you need. All other things being equal, if you have a small mouth you'll need a deeper mouthpiece. But...all other things are rarely equal!

The primary effect of volume is pitch. Big volume = low pitch. So, the volume you want in the mouthpiece relates directly to the horn you put it on (and to some extent) on the range you play in.

But wait...there's more. Pitch is also controlled by the size of the backbore. BIG backbore = HIGH pitch. So, in theory, you can raise the volume, and the backbore, and stay at the same pitch range?

Not quite. There's "slot size" to consider. Smaller volume/smaller backbore mouthpieces "slot better" than big volume/big backbore mouthpieces. This is either good, or bad, depending on your needs and abilities.

So, you have a multi-dimensional space to wander around in.
First, consider your face and lips: how big is your mouth, and what sort of rim do you like (soft and comfy or sharp and hard?). Next, consider the pitch range where you want noticeable slots. Finally, consider how narrow you want those slots to be.

Finally, note that you can tradeoff very narrow slots over a very narrow pitch range against more forgiving slots...but with recognizable slots over a wider range. This suggests that beginners (or those who are practice-challenged) playing in a narrow range may prefer small backbore, small volume mouthpieces while experienced, skilled, working players jumping from pedals to the treble clef may prefer large backbore, large volume mouthpieces. Notice how this corresponds to the usual recommendations.

Warning: the above is based mostly on theory and equations. I welcome corrections by people who both understand the equations AND have more practical experience than I do.