Today I played with my London with 5 grams of lead at 2/10 and on the throat and kept the balance the same. The results of this experiment weren't quite what I was expecting.
At first I found the added weight very welcoming. More pop and stability and my volleys and overhead smashes were awesome. My groundstrokes felt more solid on impact as well. However, as I continued playing, the negative side of adding more weight became more apparent. I wasn't getting as much spin on my shots as I expected and the lack of whippiness in the racquet really affected my ground strokes more than I thought it would. i was hitting long quite a bit because of the lack of top spin. It wasn't until I switched to my other London that I began appreciating it in stock form.
I immediately got my spin back on my serves and my ground strokes with more action on my balls. For me, the lightness of the racquet really allows me to whip, maneuver and control the racquet better than with the added weight. On ball impact, I find that whatever little instability there is, actually helps me by allowing my hand to twist to get on top of the ball on my forehand for added top spin. So for now, I'll just have to figure out how to improve my volley's without the added plow through... it's just a trade off you have to deal with.
I would say that if your more of a flat hitter then adding about 4-5 grams would be beneficial... but if you are more of a top spin hitter like me, I'd stick with the stock weight.