The 146 is certified to FL310 or FL300 depending on model. They're limited to FL280 simply because they are not RVSM capable. I think that the ASI is not corrected for compressibility and thus you have to level off approx 100ft below your planned FL and the aircraft will apparently "climb" the last bit as the speed increases.
I did a test flight once at FL310 in the 146 unpressurised. There was an almost overwhelming desire to take the mask off just to see what would happen.