True story...at a job I worked at, migrating a large customer from mailman to ListServ caused them to "lose" about 1/3rd of the list. I'd expected this knowing that MailMan's bounce handling is atrocious, bordering on non-existant (atleast at the time) - the client did freak out but after manually investigating a huge number of the automatic deletes, each and every one was an invalid address, and the old mailman lists were trying to deliver to them for years.
I've learned that for the most part the ListServ automatic delete will do the right thing when the settings are appropriate for the list, especially when combined with PROBE. It will help your deliverability and improve your delivery times.
LSoft also had or has an "HPO" license, which I believe enables more threading amongst other things for very large lists like yours. They may or may not have the HPO (High Performance Option) anymore, I haven't kept pace lately.
HTH