1. I had over 4 million polygons rendered at a decent framerate (actually there were much more, since I didn't count shadow polys). However that is just what you see, the whole scene can have hundreds of millions polygons as only the visible polygons are rendered.
2. I don't know what you mean here. If you mean mathematical calculation time to render a frame, it's completely calculated by the hundreds of cores your GPU has. Not much is calculated on the CPU at all.
3. Yes, maximum. You can do as good or even better looking games than Crysis.
4. I think the engine is very efficient, because it uses the GPU for almost everything. Of course it has also some smart logic how to process things.
5. I would do so, if you are serious about writing games. But that's just my opinion. BlitzMax can do a lot, but it's not a industrial standard. Using Leadwerks Engine, it doesn't matter very much which language you use, since the command syntax is almost the same in all languages. However, the vast support of 3rd party libraries in C++ is what will benefit you when writing serious games, like controller supports, SDL.net, sqlite3, encryption libraries, etc...
If you however feel comfortable with BlitzBasic (and thus BlitzMax), you can always make the decision later on too, or use both BlitzMax and C++ where each fits better.