When I purchased my Trail Rat 2.0 I replaced the 6v 10w bulb with a 6v 15w bulb I had in an old Niterider unit. I then bought a couple of 6v 20w bulbs for about $3 online and tried those. To my eyes, they seemed to put out less light than the 15w Niterider bulb, so I switched back to the 15w Niterider bulb.
FWIW, the Niterider Trail Rat 6v system comes standard with a 10w bulb, but can be purchased with a 15w bulb, so you should be fine with a 15w. On the other hand, the folks at Light & Motion told me their Solo lamp housing was only designed to handle the heat of a 13w bulb and using anything more powerful could damage it.
Of course as wattage increases runtime will decrease; don't forget to keep that in mind.
The 15w bulb was noticeably brighter. The 15w bulb was more of a spot beam, so it had a much brighter center and illuminated further down the road, but had less side spill than the 10w. I didn't really need the side spill of the 10w since I was using it in conjunction with the Solo 13w, which was already a fairly floody bulb.
I'd assume if you have a 10w bulb and a 15w bulb, both from the same manufacturer, with the same beam pattern, the 15 should put out noticeably more light. It's when different manufacturers start coming into play that things like color temperature, reflector quality/beam pattern, and other efficiency factors can start making the power consumption (watts) become a less reliable measure of perceived light output.