Honestly you'd gain a lot more my doing maximal strength work. Power Cleans, Power Snatches, deadlifts, squats and bench press.
Check this out: http://www.dragondoor.com/cgi-bin/artic ... icleid=153
only talks about the big three, but including power work(cleans and snatches will really make you punch and kick harder.
With all you running and martial arts sparring I doubt endurance is what you lack. However increasing max strength will make everything easy so it almost acts like an endurance increase.For example I can sprint way faster and longer after starting weightlifting, simply because it's not as hard, even though my lungs have gone to shit(I smoke way too much.)
You can do these and increase max strength and not gain weight. Just don't eat extra. It happened to me(despite wanting to get bigger
I'm guessing you may have read Pavel's Naked Warrior
he also talks about the importance of maximal strength.
Remember more isn't better -better is better.