Big power, decent throw, light weight and small is not going to be easy. If you haven’t done so already measuring your existing light with a lux meter and then figuring out what light intensity you are targeting would be a good start. The major optic manufacturers show the cd per lumen for each tested led so you can work out how many lumens you need to produce. Unfortunately I have not found a good formula to predict how adding more lights affects the throw or beam width.
Cree has a tool for showing lumen output and electrical charactaristics. If you over drive a small number of led’s they start becoming very inefficient and run hot , and as the led operating temperature increases the efficiency drops again.
http://pct.cree.com/dt/index.html
Lexel is a member here that sells a lot of drivers and has modded quite a few lights. He’s not been very active here lately but pm’ing him might be worthwhile.