The reason why you didn't get set at 15Mbps could be from a few simple reasons. If you tell us which modem you have, we can help you obtain the Transceiver Statistics from the modem which will explain why or why not you should have a higher speed than 15Mbps.
Verizon does publish some information as to where they plan on expanding FiOS. I don't remember where exactly they are, but it gives you a general idea of where planned expansions are.
Additionally, slow-downs at night is due to a lack of capacity in the CO generally. Verizon needs to bring in more backhaul or otherwise increase capacity inside of the core network. Sometimes you can be moved to less congested circuits or DSLAMs, but there's no guarantees that there is one available.
