Your ROI is all wrong. You are wanting to get BTC for BTC. You said that right now you get 4.2BTC/day and it would take you 20 days to get that back. Your looking to get back 80BTC that equates to at todays rate of $230/btc or $18,400 when you paid 80BTC at $16 you paid $1,280.
BTC is worth what it will buy and a 1 BTC today will buy a lot more than 1 BTC from 6 months ago. So you can't use BTC for BTC as your ROI. If the price of BTC stayed relatively the same then you could. So you have to use something else to determine your ROI say USD which does not fluctuate as much.
His calculation is all right. Stop thinking in USD or EUR, he invested BTC and he is getting back BTC. Same with those that invested BTC in ASICminer, they are getting paid weekly in BTC.
Those who invested BTC 110 in September to buy a Batch #1 Avalon could have had less risk by just have hording the BTC (no risk of no-delivery if Avalon failed like bASIC did, or end up with a late inferior product like BFL is trying to pull off)
The whole point of stimulating the BTC economy is to offer people the option of investing their BTC into something that pays them back in the long run (a few months) their BTC and in the mean time contribute to the growth of the network (by adding security or new features/products/sites that brings in more users)
What else could you do with BTC? Just hord it? Speculate and sell it on an exchange for FRN? You earn BTC by offering something of value and you spend those BTC on other things people offer.