This is a great site for battery questions...great tips on run-time estimation and how to chose type of battery for application.
http://user.fundy.net/cyclist/bikecurrent-FAQ.html

If you add an extra cell in series to increase voltage, there is no extra increase in actual runtime or aH. Unlike a cell added in parallel. But in parallel, there is no increase in voltage.

As for the Philips IRC lights, they are not "overvoltaged". The reflector has an infra-red coating added to it. The coating increases the filament temp without increasing power-usage. (hotter filament=brighter light)

Osram also makes an IRC, but I have only found it in the 35->50W version.