Increased speed would have a small effect on smoke production, but climbing a grade pulling a string of cars typically produces a noticable change. This is because the engine is working harder which you can see on a z4k ammeter; the current might increase from say 1 amp up to 1.5. The motor driver board senses this and serial data to the AC reg causes it to increase voltage to the heater, thus more smoke. When working properly it is a neat effect. Lionel started using AC regs in about 2003 on higher end tmcc engines, and on into Legacy engines. Arguably the AC reg boards are somewhat fragile and seem to fail often IMO.
Making big smoke is all about heat energy delivered to the heating resistor, and it takes in the range of 10-12 watts to do this in my experience. So running command at 18 volts you want to see a .6 to .7 amp load increase when smoke is switched on, especially for steamers. Less for diesels because they don't need to smoke nearly as much to look realistic.
The downside of big heat to the resistor is you really need to prevent running the smoke unit dry, lest you get charring and scorching of the wick material. So adding 10-15 drops of fluid (about 3/4" in a dropper) frequently, whenever the smoke starts to noticably decrease, is a good idea.
The real detriment of adding too much fluid is that it will overflow into the fan compartment where it will get into the motor bearings and likely trash the motor. So if you added 10-15 drops of fluid, and it won't smoke, DONT ADD MORE! The problem is not lack of fluid. Either the resistor is burned out (unlikely), or it's not getting power, or the fan is not moving air for some reason.
Just some of my observations about smoke units, and hey, I'm no expert! 
Rod
