Ah, I knew it was down there. I meant finding it located closer to the surface. My system sux for DF right now and I think a magma pump-stack would make my battered MacBook try to kill me in my sleep.
There's a pump stack design that calls for a small reservoir on each level. The FPS drop with a 4x1 stack comes from the warm / not warm status of every stone constantly toggling during pumping, and having a tile of magma either side of the one constantly being pulled from by the pump above keeps that middle tile warm.
Or you could build a regular pump stack and disable it once it's done its job. If the magma's not moving, it doesn't affect FPS.
Another option is to put your fortress closer to the magma. You can either do this on a regular world, either weaving through the caverns or building into and then walling them off, or you can gen a new world. Increasing the number of levels above layer 4 will put more dead space between the lowest cavern and the magma, giving you solid rock to carve into. Since the rest of the layer options are already set to minimum, you'll need to reduce the number of caverns if you don't want to gain too many z-levels overall. Removing one layer shouldn't affect things too much, although if I've understood the wiki correctly it'll lead to a big jump in difficulty between the upper and lower cavern.