I've got over 10 million acs_objects currently in a 4.2 Oracle system. I don't have that many users but I do have over 1000 site nodes. We've had a few performance problems, but they have all been resolved when they occured without changing the basic data model.
I think if you are expecting 5 million max then you don't have much to worry about. I'm expecting 50 million by the end of the year and the only real problem I have right now is the time required to import the object_context_index table. I expect to solve that by switching to rmon for backups.
The advantage of the ACS data model is not speed but flexibilty. If you are even considering ACS then you don't want to sacrifice that until you have to and I suspect that time will never come. Also don't underestimate the value of one sequence number accross all objects and tying everything together with rels. You can dig alot of information out of the database in very general ways. This is very useful for admin and debuging pages. The only drawback here is don't lock the acs_objects table.
So to answer your question if you are using ACS you can't over use ACS objects. The consequences of not using them are far greater than trying to over use them. If you are certain the data model will be too slow then you might want to consider a different toolkit (or a faster machine)
