Let me know which scenario we're looking at.
Right now I cannot test on my home PC (on which the original number was based), but on my laptop I get about 960mb of ram filled with 1mil. objects. If I do drawing (especially GL3 which has batching) I get up to 1.1gig. With 1.2mil. it takes about 1.15gig and with drawing it goes up to 1.4gb. But my previous test also used a simple logic as well which could increase the size. Also, I don't think it crashes only when going pass the 2gb mark as reallocation can fail way before that.
So basically right now 1 instance takes quite an amount. On VBO side (which also takes like 300mb with 1.2mil. quads) I could just set an upper limit and then make it draw before batching is done. Normally you wouldn't draw 1mil. things on the screen at once and even if you do, then normally you wouldn't be able to batch all of them. The same with instances though. So while it would be nice to show an error or something (at least in debug), I don't think any real change is needed.
I live Pennsylvania, today is August 9th 2013 at 8:51 P.M.
If only the ordering is changed, then I am alright (although it still could be pain in the ass when needing to revert). I just feared for one minute that the changes are also done in reverse and that would fubar everything.