GHS/BTC = 0.00747 which is the equivalent to THS/BTC = 7.47 = 4.487 USD
So you need to pay more than 7 BTC for one TH/s? It does not make sense at all. Here's why:
Currently one TH/s generates BTC 1.12 per month = 667 USD.
Don't forget maintenance. One TH/s costs 260 USD per month, reducing the profit to 407 USD, so it takes 11 months to break even. With difficulty increases it will surely never happen.
Am I right?
If you buy a mining contract and hold it, then it probably won't be profitable because you will end up with less bitcoins than the amount you put in. If that is your goal, then I agree it would be better to simply hodl the bitcoins themselves and your calculations seem to support this conclusion.
What you are forgetting is that you can profit by trading the mining contracts themselves.
The worst would be to reinvest your revenue in GHS because you will get a fraction of a fraction of the BTC you had
Also there is a risk of losing all your coins if the site closes