First, some background info about the bugs:
PA is gram negative rods
Cepacia is gram negative rods
Staph is gram positive cocci
TOBI is formulated to be most effective against gram-negative rods. If anything, it's going to be less effective against the staph infection than the PA or cepacia.
Staph is sometimes difficult to treat because its protective "slime" can be 20 times stronger than other bacteria. Also, when staph carries out its metabolic functions (remember, bacteria are living organisms!) the byproducts can be infectious as well. Beta-lactams are effective against staph. Merrem is a beta-lactam, and I have had good response with that one. Usually my FEV1 really jumps after just a few days on Merrem. Also, beta-lactams are being used along with tobra because of the beneficial synergistic effect they have on wiping out strong bacteria. Cycling TOBI would not make the cepacia grow more over time.
There is a minimal risk of developing tolerance to the meds or causing a panresistant bug when the proper antibiotic is correctly used against sensitive species of bacteria. This is why it's important to have a sputum culture to confirm that the bug you're culturing will be targeted by the prescribed antibiotics.
Although there might not be a DIRECT link between TOBI and improved FEV1, it's reasonable to assume that by keeping PA and cepacia infections under control, lung health is better overall.
There are studies being conducted right now on a variant of Ceftazadime (a cephalosporin), which is hoped to be even more effective against cepacia. The product, called Aztreonam is ahead of schedule in its testing phases. It will likely be used in a cycling mode like TOBI.
Right now I mix my own Ceftaz (Tazicef) and alternate between that an TOBI. This is off-label use of it. Once Aztreonam becomes available I'll use that instead if my doctor thinks it's a good plan.