The nVidia panel or Profile Inspector can both do "1/2 frame" stuff, and cut thing in half, and/or allow you to set a multiple that gets you close to 30fps, but it is probably best to leave sleeping dogs lie, indeed...
"Normal" running out in the middle of the ocean on my machine will yield 140+ fps routinely. Convoy encounters willl drop me to roughly 80fps, and when inside a convoy, I might go to 40fps. But what seems to really impact things is when they attack me, and I'll see down into the lower double digits at times, and even saw 6-8fps more than once... but it will fluctuate during the ASW counter-attack. All mods and stock seem to do that. Too much "thinking" going on, I suppose... triganometry and "chance"...