The side effects are these:
Ships would take longer to turn and longer to stop, or to increase speed. That
MIGHT result in:
a) Ships colliding with eachother in convoys when they are taking evasive maneuvers, cos the AI is stupid.
b) Ships running aground, cos they are scripted to stop/turn at certain points, and those points were calculated on base of their current speed to turn/stop. The whole campaign would have to be re-checked.
c) Ships colliding with eachother during turns in the waypoints. Believe me, when they are turning from one waypoint to another, they do a pretty good mess already, cos they are very dumb, imagine with slower movements.
The good sides of doing it:
a) Ships will finally collide with eachother, it does never happen ingame, and sure happened a lot in RL.
b) Ships running aground, FINALLY, due to malcalculations in route and speed.
c) Ships won't be able to dodge torpedos as easly as they do in game, cos they would take longer to accelerate and turn.
d) The whole thing would look more realistic, as in real life ships take eternity to stop and turn. I was testing the titanic, it goes from 30kts to full stop and reverse in less then 100 meters, less then 10 seconds. In my head that's a joke! It would be more realistic if it took 1 km to stop, at least.
The mod itself is very easy to do, and the things it would break is what I've pointed above.
I will do it, test it, and release it for the weekend. Screw the side-effects, who want it, can download it. Who don't want it, can complain about it and tell me to go to hell. I couldn't care less.