This is a great site for battery questions...great tips on run-time estimation and how to chose type of battery for application.
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.