Given that you would have to shave a lot of metal off, I wouldn't even attempt it. The largest aspect of the Cobra would have to become smaller than the currently smallest part. Then there's the fact that the cap would no longer fit, shaving Ti is no small feat, etc.
Far easier to just custom make a new sized tank I would think. Phiniac would need to get permission from Siam first, and if I recall correctly Siam already stated that they are to be used with their glass only.
Yes. It would only change the whole design and I. Fact, it could make it not functional. And if it even worked, it could change the dynamic of the air flow.
BUT, here is what I personally would suggest. Order a replacement customized tank. And order one plain "just in case" tube. What are they? $10-15?
OR, just go and order two customized and you can change up your style during cleaning.