I researched this for my gs500 which has a pair of 1157 bulbs, and I see that the S40 has a single 1157 so this is the easiest sure fire bulb upgrade there is.
The standard bulb in the GS is a 32/3CP that's the ratio of the brake light to the tail light as you can see it's about 10:1. My GS had the original Stanley 32-3CP bulbs which are probably the best standard 21/5W 12V 1157 bulb, I compared them to the GE Nighthawk 21/5W 12V 1157 which they claim are 20% brighter than standard bulb. My finding was there there was no difference to the standard Stanley 32/3CP bulb.
However Stanley also make a 43/3CP bulb that is 21/5W 1157, the CP stands for candle power. So you can see the ratio is more than 14:1 the running light is the same at 3CP but the brake light is stronger which is what I wanted. It took me a while to get them but when I put them in they were indeed brighter when the brake is pressed in.
These are the bulbs
http://www.ebay.com.au/itm/272047902846 so if you want the brightest incandescent 1157 brake light these are the ones to get.
As an aside I also tried the Philips 1157 P21/5W Red X-tremeVision LED to see what that was like, it's a single point LED with two leds in the holder. This will behave as close to an incandescent point light source as you'll get at the moment. It worked very well, but I rejected it because although the brake light was as bright as the Stanley 43-3CP, the running light was way brighter than the standard incandescent, and I rejected it because there was not enough difference between the running light and the brake light, but for people who mainly ride in the day, this may be a good option but you do not get the difference between the running and brake lights, so bear that in mind.
http://www.ebay.com.au/itm/Philips-1157-P21-5W-Red-X-tremeVision-LED-Exterior...