I'm new to SH3/GWX and don't have a great deal of experience, having just completed only my third war patrol in 1939. However, my experience so far is considerably different than WilhelmShutlz's. I've enabled most all the mods that came with GWX (excluding the French fix) and am running SH3 Commander. The 16k visibility mod seems to work well for me in that my watch crew has been able spot ships before they get close enough to attack me. were they armed. I've got SH3 Commander setup to allow x1024 time compression (TC) max, which drops to x8 whenever a potential threat is detected. WilhelmShultz didn't indicate if he's using SH3 Commander, but I strongly recommend it. It can make all the difference in the world in survival and success versus the alternative, particularly if you're using TC.
I had an instance last evening where
I couldn't get a visual on a blacked-out transport, but my crew had detected him...and he had detected me, judging from the blinding spotlight illuminating my boat. Of course that made the deck gun crew have to wear sunglasses at night
but there was no doubt in which direction they needed to aim and fire. Result: first of four kills on patrol #3. So, visibility and detection in GWX seems to work both ways, but I haven't found it to be inequitably so.