It's important to note that some of the higher end and higher mAh batteries will really push the envelop of voltage. For example I've found Energizer Ultimate Lithium AA's typically run about 1.9 volts. Indeed most AA batteries measure way over the 1.2 volts they're typically suppose to put out. What's more if you've read up on NiZn AA's (the latest greatest rechargeable technology for high demand devices like digital cameras) you'll note that people regularly report that these push the voltage envelope for a typical AA. Some have even reported damage to their devices.
That said most lights and most electronic devices are now regulated so they are not affected by the increased voltage. This goes from digital cameras to the latest headlamps and flashlights. Occasionally though you'll find a light or device that's not regulated. For example I have a favorite touring headlamp that puts out about 136 lumen. I've noticed when I put energizer ultimate in it gets much brighter. In extremely rare cases because these rare and often cheap unregulated lights loose efficiency as they get brighter they will actually drain batteries quicker. This is however, as mentioned, a very rare scenario.
Needless to say I think you're plenty safe enough assuming that a 2500mAh battery will last longer then a 2000mAh... and not make the light any brighter... unless you have one of those rare cheap lights with no regulating circuitry.
== LED lights for touring ==
@mechBgon, nice gif, good way to measure longevity, I think I might be able to set up my camera to take videos like this without wasting too much memory if I can set it to a low enough fps. BTW, do you hang out on Candlepower Forums?
I'm curious if you or anyone here has any experience with the Fennix HP10, River Rock 6V Headlamp, or the my most recent acquisition for touring, a 176 lumen headlamp that runs on 3AA from a company called "LED Lenser" that I picked up at Lowes for $50. While I have absolutely fallen in love with it I have yet to thoroughly longevity test it and still have some concerns about it's weather / waterproof-ness / durability (since I do a lot of winter and wet weather riding) but so far I'm hugely pleased with it's brightness, longevity, lite weight and it's incredible features which include a beam intensity focus and dimmer slider.
It's amazing how bright, efficient and cheap LED's are getting... and how fast their making it into mass consumer lights. I bought my 136 lumen River Rock headlamp two years ago at Target for $25. It puts out 136 lumen for 5 hours on 4AA and rideable light for 8-10 hours. That kicks the snot out of most specialty lights that are many times the price.
== why cheap mass market lamps rule ==
I use these cheap mass market headlamps because the innovation in LED brightness and efficiency is happening so quick. They are extremely bright, efficient and effective. Most importantly I can't deal with rechargeable proprietary batteries used by specialty makers when I'm touring. I do a ton of night riding. Indeed I do a lot of winter riding and a lot of winter touring when daylight is short.
These mass consumable (and usually cheap) lights are usually not that high a quality, but they're cheap enough I can and often do mod them for my specific purposes. Examples include waterproofing with silicon sealer, creating external battery packs and creating mounts for handlebars, forks or helmets.
== externalizing battery packs on headlamps ==
One other extremely important tip I have is to externalize the battery pack on lights for winter riding.
By simply running a couple wires out of a headlamp (I prefer to use RCA cables / connectors and $1.97 4AA enclosures from radioshack) I can move the battery pack into my clothing so my headlamps are completely unaffected by the cold. It's usually very simple to connect wires into the built in battery port on a headlamp, the trick is resealing the headlamp so it continues to be waterproof / water resistant.
== secondary recharging power packs ==
On a related side note I've been reading up on resisters. Apparently it's become extremely popular for people to make external secondary battery packs for the iPods, mp3 players, iPhones, Android pones and other smartphones which are well known for being ridiculously power hungry.
The key is that everything is standardizing around USB for charging and syncing. This is a great thing since one can theoretically make a secondary battery pack that will power multiple devices. The key element to these simple battery packs is a resistor. For example Radioshack sells a 5-volt resistor for $1.47. This resistor with theoretically knock anything up to 35 volts down to 5-volts which is the USB standard for power. Therefore it's theoretically possible to create a little black box or battery pack that you can connect anyting from a 4AA battery pack, to a 9-volt or a 6-volt to and get clean consistent power to your light, camera, mp3 player or smartphone. Been meaning to build one, but haven't had the time yet. I guess I've been to busy reading / writing on special topics on forums and mailing lists passive aggressively.
== candlepower forums website, an excellent resource ==
One last tip I've found the Candlepower Forums to be an excellent research tool. Indeed I've found a tremendous amount of research and discussion on the Fennix lights and the LED Lenser headlamp I got from Lowes. It's a good place to find out what's hot and why. The variants of what people are looking at from expensive setups to cheap mass consumables is pretty impressive, but what makes it really cook is the amount of people who really know there stuff and thoroughly test their lights. For me it is to lights what the backpackinglight forums are to ultralight gear.