I wouldn't call it equivalent, but I would call it a substitute. You can't call it equivalent because TOBI was designed to be inhaled, tobra is not.
There are some people that cannot tolerate IV tobra inhaled. There are some (myself) who now cannot tolerate TOBI. It's definitely worth a call to your doctor. There were many of us that relied on it in the 80's/90's before TOBI was available and there are many parts of the world that STILL use IV gentamicin inhaled (I like to call Gentamicin, Tobra's cousin) because gentamicin is more readily available than tobra for some reason. My theory is, if it works, use it.