Faisal,
The performance drop of the current TP adapters below 5.4GHz is not a big function of gain but more of VSWR and Cross Pol Isolation performance. Gain below 5.4Ghz drops about 1db in the Hpol and 3db in the Vertical. The big difference is Cross Pol Isolation drops much more in the lower band as well as VSWR goes up to an unacceptable level for us (more than 1.8). So you are correct, the antenna does not just stop working, it just doesn't perform as well as the rest of band which it’s tuned for.
BTW- We are working on our next generation of adapters which will have full band coverage, with no compromises. Stay tuned....