My daughter has been taking flute for over 3 years, and is going to start taking oboe lessons. My daughter's instructor didn't have a strong opinion one way or the other on the low Bb vent key option on the Fox 330.
It doesn't change the shape of the bell though. The bore is still the same. If you are having troubles with your E's and G's something else must be wrong. As long as the low Bb vent is sealing then it shouldn't affect any notes besides the low Bb.