Stencil shadows are possible with A6 Commercial or higher. I think they perform okay, but don't count on them being the fastest thing in the world - they aren'nt. As far as I understand it, A6 is not the reason for stencils not rendering that fast, though.
"Max polygons" is no value you could really ask for regarding modern game engines. The number of polygons that are viewable while still having a decent framerate totally depends on what you do with them: Are they textured or not, do they use shaders/FFEs, are they dynamic entities or static meshes etc.
Max. light sources is currently 8 lights at a time (not in the entire level!). Dynamic lights, that is - you can have an unlimited number of static lights. One of the next versions (currently beta) will support a new lighting management which allows for more dynamic lights.
As for shader problems, I don't know. Problems with shaders are usually related to the shader code itself rather than the engine.