I play with the stock TMO 2.5 AI. It is tough, but not superhuman. I am not prepared to say it is not realistic.
You can sneak up on it at night. I managed to get within 1500 meters of a small TF, 1 CL, 2 DDs on the surface in Dec. 41 before they spotted me.
In daytime, they will spot you almost as soon as you can see them, so you pretty much have to dive as soon as smoke appears on the horizon.
Spotting is influenced by: light, sea state, speed, visual aspect, etc., so if you are running at flank speed parallel to an escort in daytime in calm weather, you will be picked up from much farther out than if you sneak in at night, slow speed, perpendicular in moderate waves, etc., etc.,
In SH5, there is also another modifier depending on whether the AI is on "Passive" or "Alert". "Alert" kicks in when any AI in the area spots you, a torpedo or something explodes. I have not been able to figure out if this is in SH3/4 as well.
Underwater, you can escape if you go deep and slow, only going to flank to escape DCs. I have been sunk or heavily damaged, but usually manage to escape.
TDWs IRAI AI in SH5 is a lot deadlier.
