ASICminer is publicly traded, but only on Havelock. That's about as good as it gets so far.
Wouldnt want to invest in a hardware company. Those are directly influenced by the value of BTC. In that case i can invest in BTC just as well.
An exchange or payment service grows with adoption as a payment service, it doesnt really matter how much a coin is worth, just that "a large amount" of fiat equivalent actually gets used.
Doesnt really matter if a BTC is worth 1k, 10k, or a 100 dollars, the more people actually use btc to buy stuff the more those companies have to grow (since they take a small cut of every transaction).