Basic math shows how overpriced the shares are: there are ~10million more bitcoins left to be mined over the next century or so. Even if ASICMiner were able to maintain 20% of the total network hashrate throughout that period (breathtakingly unlikely), they'd only mine 2million BTC. Split between the 400,000 shares that creates an upper limit of 5BTC/share.
Not necessary. A significant part of dividends come from selling mining equipment. Because of that, the real profit they are making could be 30% or even more while it has 20% of the total network.