I made a test BoE mission 15vs15+1 capship thing. I had every single detail up, and my computer was rendering every single poly. Add that with AA and AF, and it was STILL running well! You seriously underestimate the power of Freespace.
Also, the best way to do a BoE mission is have a bunch of capships, and assign a few waves for the cruisers and corvettes so that the action is prolonged without the loss of performance. Also, only 6 fighter wings would be needed. Since you would usually be around those fighters, 24 fighters around you is enough. Besides, you could just set the wave count to be high, and have the fighters get launched from near their destroyers, without a warp-in effect. When done right, you have great performance in a very large battle. Setting a high inaccuracy rate for the powerful weapons of the capships is a great way to prolong battles, too. To make the users go into dogfights without dying every time, wingmen with a powerful AI and a lowish wave count, versus enemy fighters with a crappy AI and a high wave count is a fun way to keep the player alive and busy.
I'l shut up now.