I've been overvolting MR-11 (only difference is in diameter of the reflector) lamps for years now without any issues. I use either 6V overvolted to 7.2 V or 12 V lamps overvolted to 14.4V. Either one gives out the same amount of light and it all depends on how many batteries I want to carry. I use 15W and 20W bulbs but I use up to 4 lamps at a time.

My preference is to use RC car batteries (3.3 ahr or 3.8 ahr). If you wire them in series, you get a 14.4V battery and if you wire them in parallel, you get the same run time for a 7.2V system vs a 14.4 V system. One of the reasons I run both is that in early spring I don't need the run time of the 14.4V system and so I can carry only one 6 cell pack (much lighter). It just allows for more flexibility. They aren't 5 ahr packs but I can get a bit over 2 hours on a 20W bulb which is enough for my needs. If you don't need the whole run time for the 5 ahr pack, these are a less expensive option. These guys have them for pretty cheap (as little as $19 per pack). They also have a smart charger that will charge up to 16V for $25.

Rather than carry the batteries in a water bottle, I have Rocket boxes that I got from REI that work wonderfully for my packs. That way, if I have to fix the batteries for some reason, I can take it apart rather than have to cut the bottle away and refoam them.

Here's some pictures

A 14.4 V pack, wired in series



The batteries comfortably nested in their boxes



The battery boxes taking up the water bottle cages I don't use in winter time anyway.



Hope this helps