a single small (500ml) water bottle weighs about 1lb. Two full 750ml bottles will weigh around 3 lbs
i dont think most people notice the difference between a full and empty water bottle...
Honestly, I think most of the improvement will come from riding more and bulking up my chicken legs.
i recommend riding a REALLY heavy bike.. then your bike will seem fine. I started riding a pedicab, and now every other bike i ride feels light as a feather