A look at old CAA stats indicate that when Air Wales were operating NCL-PLH (excluding the stop in CWL) they were generally carrying around 2395 - 2798pax per month, in the summer (although 6G operated twice daily).
Even if WOW achieve the lower end of this, say 2400, that equates to about 40 per flight and this doesn't include the additional pax likely to fly to NQY.
....not bad on a 50 seater aircraft!
I think if they can achieve these figures, WOW should do well on these routes