But I see you're using the lazy-man's big-box method:P ... If it´s there to plug a leak it will definitely hurt performance.
I decompiled the bsp to map and found that some of the floorbrushes underneath the water have the 'window' flag. That's probably the reason for the leak. If you want send me the .map file and I'll make it leakfree without that box and clean up some of the brushwork without changing the appearance.
seems the performance is pretty bad. So whether or not you set more stuff as detail it will still have the same performance problem, but it will help reduce vis-time. Best thing to do is fix the leak and get rid of the outer box (and use a real skybox instead of black brushes, just go with space1 or custom sky?).
..correct me if I'm wrong but I think you use the Brush Subtraction tool too much.