To be profitable at current prices the difficulty should increase no more than ~5% each step...
I think it's even worse. At current MINING price (lets take 0.027) and an increasing difficulty of 5% per step it will take 13 months to be profitable (roi on dividends). And if difficulty increases by more (it probably will) you'll never earn anything...
At a 5% increase per step (repeated indefinitely), MINING will earn 21 times its current biweekly dividend in total (sum of a geometric series), which starting tomorrow will be 14 x 0.13 mBTC, so 1.82 mBTC per difficulty readjustment period (approximately, if difficulty rises, the readjustment periods will be less than 2 weeks) for a total of 38.22 mBTC.
With a 10% difficulty increase per step, MINING will only ever pay back 20.02 mBTC.
edit: updated sloppy math