I don't have a clue why Lionel used the 27 ohm smoke resistor for command locomotives, I understand it for conventional running as they apply full track voltage. For TMCC, you get a half-wave rectification of the track power minus a diode drop from the triac on the smoke unit. If you boost (hold down the Alt1/9 key), you get the full track voltage. Don't do that too long! 
I don't know of any issues that have come up from the mod, I've done it a bunch of times. I did some tests and measured the temperature rise of the smoke triac on the R2LC with the 20 ohm resistor to insure that the change wouldn't adversely affect the life of the electronics. In all cases, the smoke triac had no more than about 25C temperature rise over ambient, which is well within it's ratings. The current goes from around 1/3A to somewhat less than 1/2A, also well within the triac ratings.
The resistor change brings the wattage dissipated across the smoke resistor from around 3 watts to around 4 watts. This is in line with what newer Legacy locomotives dissipate in their similarly sized smoke units, so I don't see any major issues with heat. I can't imagine this would affect the smoke unit motor, I suspect other factors will kill it long before the slight increase in heat.
Some folks have gone with as low as 16 ohm resistors in the smoke unit, but I think that's a bit much. That brings the wattage over 5 watts and also raises the current to over 1/2A through the triac. With that resistor value, I had a 50C temperature rise in the smoke triac, and things were getting a bit toasty around it.
I did experimentally add a heatsink to the smoke triac with the 16 ohm resistor, and that helped with the heat dissipation, but it still ran over 40C temperature rise. You could probably get away with that, but I choose to be a bit more conservative.
