I'll tell you what David.  I'll find that BR4 or 5 I have that's over a gig, load it render it and I'll get back to you on how B7Pro handled it as far as loading, rendering, etc.  Also on other renders I've done to save time and resources in Bryce (and i know many others have done it) is put half your objects in a scene, render, save.  Then do the other part of your scene, then blend in Photoshop to look like the full render!   Setting priority to high in Bryce helps render times as much as 50-80% as multiple processors are utilized BUT it also may help in the loading and structuring in a scene too, but not much heard about that!  Let me find that file, i'll get back to you David.