Just thinking about removing a gas tip tube to maximize headspace. What if I just slide its o-ring on the post? That should in theory keep the keg sealed. What do you guys think?
I've not done it myself, but some people cut their dip tubes for various reasons. Sometimes the gas tube and sometimes the liquid tube. Removing the gas tube entirely is likely to cause a gas leak . There is a small shoulder inside the post that pushes against the lip of the gas tube to "sandwich" the oring against the male keg post and the top of the dip tube. This is what keeps the gas in the keg.
If you're wanting to remove them due to concerns about beer being sucked back into the co2 tank, there are check valves available which can be installed on the manifold or gas line to guard against low pressure issues. I think this would be the best remedy since it also protects your co2 equipment if you overfill the keg after altering the gas tube anyway.
The other reasons I've read about for cutting the tubes seem silly to me (more room for beer, less trub, oxygen blanket when purging) but that's just my opinion. I would vote that cutting is the better option than fully removing the gas tube however.