I think the point is being missed here with all of the "posturing" about whether flywheels are useful or not. The original poster explained he has an HOn3 C-25 loco he's working with. Now this thing isn't much larger than an N Scale boiler so the flywheel will have to be very small to fit in the loco, we aren't talking about a Hobbytown E unit here. So how much of an affect would it really have, about as much as the size of the flywheel or almost none.
Secondly, and everyone seems to want to choose to ignore the obvious here, these things aren't balanced, again, very hard on motor bushings.
Third, flywheels were introduced back before todays high torque "can" motors such as the Sagami's, they worked great on the cheap motors they were designed for, open frame 3 or 5 pole motors. They worked well removing the "cogging" action of these motors, although again they still are not balanced.
With todays super smooth new motors and good gearboxes flywheels become a mute point except to make a crappy running engine "appear" to run smoothly. It still come down to having a good motor, a good gear box, and the entire mechanism being tuned for smooth operation.
All he really needs in that C-25 now is as much "weight" in that thing as he can get into it possibly including the tender to insure good power and good contact. He has the biggest part of the equation soved with the NWSL motor.
Again, if you have a crappy running engine a flywheel will help considerably, but that's more of a cover up than a solution. Sort of like the old joke of putting lipstick on a pig, but you still have a pig, that doesn't change.
Mark