You're on a much different level of usage than I am. I've been running a Commerce version of ImageFolio for something like three years with about 30K images currently in the archive. I use it to archive images for my commercial clients. They can access private pages and view proofs archived from shoots do for them. They can search and order from older sessions and get additional copies (for a fee) of images they've already purchased but may have lost or damaged.
Imagefolio has been pretty much flawless in terms of operational stability for me. I have it on an in-house OS-X server. I assume you must be running a MySQL version. I'm not. The Commerce version has not been updated at all during the time that I've been running it though Biz Design has occasionally made noises about something coming. Nothing ever seems to. I've basically resorted to hiring someone that knows perl better than I to do minor mods that I've needed.
I learned about and purchased Lightbox (Enterprise) based on posts on Imagefolio's own forum. Those posts are still there... months later. That doesn't tell me that Biz Design is terribly interested in the ImageFolio product. Here's an example
. There are many other similar threads.
I'm still working on setting up my Lightbox Photo site so I'm no expert. But so far I'm very impressed. All of their software is php and MySQL based so performance is quite good. I've only played around with relatively small test sites so far so I have no great concept of how it will handle large numbers. My sense is that it should do well. On one test version I managed to import my entire Imagefolio library in a few mouse clicks. With a little better planning I can see how I can move the whole library over without too much effort it that's what I choose to do (I probably won't for other unrelated reasons)
Be sure to email Lightbox for an admin demo. That's the part that sold me. Also look closely at some of the sample sites running their software to get a better feel for the possiblities. The developers are quite responsive to pre and post purchase questions. Describe exactly your concerns to them and they'll give you straightup answers. Pretty much what I did and it's how I ended up buying into it.