As you correctly point out, the return on your investment is not so viable if your production is low. Whilst wind resource is a very important factor, so is having a productive and reliable turbine. I suggest you have a look at the turbine production data on this site (link from the home page), as this is very revealing!
Also bear in mind, that as of 1st April, ROCS will be doubling for small turbines (<50 kW) and this will be a big help.
In places like Orkney, Eoltec turbine owners see anything from 16,000 to 26,000 kWh a year. With a 10p N power export tarrif and single ROC's at ~4p (in the hand) at present, thats anywhere between £2,240 a year and £3,640 a year. When double ROC's come along, these figures become £2,880 and £4,680 a year respectively.