The question of how much overdrive is safe for a bulb is actually far more complex than 'Multiply by 1.25', although for many applications a 25% overvolting will be acceptable.
Basically any sort of overdrive for an incandescent lamp will trade increased brightness and efficiency for decreased life.
http://www.walamp.com/Product_Pdfs/isl297b.pdf has equations for estimating the life change for lamps when you operate them at different voltages. Basically power goes up as voltage^1.55, but light output goes up as voltage^3.5. So if you run a bulb 'hot' you will get many more lumen for each input watt. However life goes as voltage^-12.
What you need to do is decide what tradeoffs work for your application. A 25% overvoltage means 41% more power consumed, 118% more light produced, but a life that is only 7% of full rated life. Many people use this level of overvolting when they use 4000 hour halogen lamps for portable applications.
On the other hand, if the bulb as designed already has a very short life (say 10 hours), then a 25% overvoltage is probably silly.
Finally, you must consider the startup surge aspect. When a bulb is first started up, the filament is cold and filament resistance is very low, about 10% of normal operating resistance. This means that very high current can flow, potentially damaging any weak spots on the filament (this is why bulbs often blow on startup). Overvolting makes this problem worse.
-Jon
