How noticeable is the performance difference between ESC's with different on-resistance? For example, Novak Havoc with .0012 ohms versus Havoc Pro with .0004 ohms. Might the answer also depend on motor current demand?

It depends on your skill level, and what type of racing you are doing. Racing offroad, you wouldn't notice a difference. If you were racing onroad, most likely oval, and you are capable of turning consistent lap times, then you may see a difference. If you were to run a modified motor, the one with the highest resistance would be warmer, and also may be limited to how low of a wind you could go.

With todays ECS you won't notice a difference what so ever. You'll get more resistance if you put 5% more solder on you solder posts.

Bang on the money there. How many people worry about things like on-resistance when there are so many other loose links in the electronic chain! I'd say good wire, good soldering, and quality brands make ALL the difference

Thanks for the feedback all...looks like the most bang for the buck is to buy a faster motor to use with current ESC. I'm only talking 17.5 to 13.5 after all.