If you already purchased the Reflectalite GH88 bulb, instead of overdriving it, why not try it out with
2 cells (either D alkalines short term, C rechargeables with an improvised metal spacer, or D rechargeables if you have any) and see what it looks like. Running at spec voltage it's still higher wattage (4.2watts) than any of the other bulbs mentioned so far, including the H-26P.
If you've already tried this and it didn't live up to expectations, then you may want to try the "
absolute brightest possible 3-cell PR bulb" which is the Carley 606 (3.7v 1.7amp 6.3 watt) which they market for rechargeable lights. I know of no easy way to obtain one of these other than walking into the shop in Los Angeles; but I noticed you are in Bakersfield, so possibly you might get to LA sometime.
The next brightest option is the KPR139 (3.85v 1.32amp 5.08watt), followed by the KPR130 (3.85v 1.2amp 4.62watt). These 2 bulbs can often be located at wholesale lighting supply warehouses for less than $2 each, thereby avoiding on-line shipping charges.
I have included links below to all of these bulbs for reference. If Al Bundy doesn't come back to this thread, hopefully this information will at least benefit any future inquiries into this most basic of mods.
Carley 606
http://www.carleylamps.com/star.htm
KPR139
http://www.servicelighting.com/EiKo-40057-KPR139-B3-5-Miniature-Incandescent-Light-Bulb
KPR130
http://www.servicelighting.com/EiKo-40055-KPR130-B3-5-Miniature-Incandescent-Light-Bulb