As far as I can tell, Emirates averages sales of around 5 out of 7 seats in F on it's B777s and 10-12 out of 14 on it's A380 flights.
So that's around 16 F class PAX out of Perth per day.
In addition Qatar has capacity of 28 J seats per day and doesn't offer F from Perth.
Given QR's marketing strategy I would assume some of those PAX would fly F given a chance.
Etihad's 3 class 789s have 8 F suites.
I wouldn't think it was unrealistic to assume EY could pick up 6-8 PAX per day from EK, QR and other airlines which don't offer F from PER.
Their F product is very good. The 787 doesn't have a shower and a bar like the EY & EK A380s but I'd rate the suites, the food and the service above EK.
So the EY B787 product could certainly compete with the EK B777 product even if it struggled against the EK A380.
Also the flight from PER has excellent connections to flights to LHR CDG and other European cities, with only 2-3 hours in AUH.
The apartments on the A380 AUH<->LHR would be a drawcard as well.
As you observe EY manages to sell a lot of upgrades at the airport.
So they could probably fill any empty F seats by pushing their top tier FFs up to make room in J.
They do this frequently on the CDG<->AUH flights.
Of course the fact that EY hasn't gone down this path suggests that I might be wrong!
(I've arrived at the load figures using both ExpertFlyer and industry sources but I don't pretend to have actual knowledge of EK loads.)