Hey amckern, seeing as you're working on a modified version of the p15 tools, I would really appreciate if you could answer a question for me. I'm having a lot of trouble with my leaves (97.4 fullness and about to go right over) and I need to know why func_walls generate leaves during bsp. Also, what could I use alternatively that wouldn't generate more leaves? If I made a duplicate of some geometry, made it a func_illusionary and textured the original clip, would that create the effect of a func_wall without generating the leaves?
I just did a Google search and found that one of my fellow coders has an in-depth look at Vis - http://www.egir.dk/index.php?page=hlvis/hlvis.htm
Look at your map, and see if you can do anything to reduce the leaves, such as cutting the brush with clip, and not letting vis do it for you - it's a lot of gl_wireframe 1, but well worth it once it's done. Also, null any sides of entities the player won't see.
Making a square of brushes around a pipe or such will reduce the entity count and also make the leaf count smaller, speeding Vis up, as you can see by the MS Paint shown here.
Example of Brushwork
At the end of doing this, you should have a better map for Vis, which makes fewer leaves and speeds up the compile process even more because you do some of the grunt work that Vis would normally do.