The price 0.001 btc / GHS sounds a bit too good to be true IMHO. You'd better be careful before buying hashrate from it.
Whether it is a scam or not is moot as the user will still end up losing money.
20% average difficulty increase is a good estimate.
https://bitcoinwisdom.com/bitcoin/calculatorIMO, 20% was a good estimate for the period before June 2014.
But the difficulty increase has slowed down since then. The increase rates from July 2014 onwards are 3.08%, 8.08%, 5.30%, 20.86%, 15.03%, 8.75%, 16.20%, 0.98% and 2.81% according to
https://bitcoinwisdom.com/bitcoin/difficultyWill it speed up once again? May be. Will it go back up to 20% every 12 days? May be.
But it is hard to predict.