Firstly, GWX's graphics are as optimised as we can make them while keeping image clarity. Ship skins are at 512x512, Aircraft are 512x512, the player boats are 1024x1024. So, you could try to reduce them further but you will lose quality.
The biggest FPS killer in SH3 is AA & AF. AA = The smoothing of jaggies & AF = Quality at distance. Now, if you don't like jaggies, your only option is to reduce the AF which will help alot.
On most GFX cards there is 4 stages of AF and if you imagine breaking the SH3 visible environment into 4 distinct stages, close, close medium, far medium & far, can you understand the rendering GPU usage with AA turned up and AF on full. Effectively meaning no jaggies & best texture quality on all objects viewed/rendered at the furthest distance you can see. So, pull it back a bit on the AF.
The effects of reducing AF are most noticable when using the F12 view and at high altitude, however it's hardly noticable on the bridge view.
Now, just to throw this into the mix, there is a very strange occurance in SH3 that when looking (even in open sea, with no other vessels) at a certain bearing (in U-Boat or not) there is a very noticable FPS hit. For example, I can run at full AA & AF with 100-120 FPS but if I look to the horizon on this specific bearing, my FPS will drop by 20-30.
It's even worse when rendering 15m/s winds & high waves. Many people have tried to track the reason down but nothing has ever been found.