Whether it would be brighter would depend on the bin of your current luxeon. It might not be worth the time and expense. As for overdriving an L4, I'd be careful. The stock L4 generates a lot of heat as it is and it's underdrving the luxeon at around 660mA, in stock form.
If your L4 is bright and has a good color tint, I'd leave it alone and think about putting the new 5W lux in a new light.