Generally not. There are fixed and per-unit components to field...

Generally not. There are fixed and per-unit components to field operating costs but when modelling gas projects it's usual to use a constant opex. Keep in mind that the two new wells will go into decline almost immediately. Running a depleting gas field is an exercise in treading water, you have to keep drilling to maintain production, so per GJ opex estimates are based off a long term average of production rate.

In reality, yes the higher rate will mean slightly lower opex because fixed costs like staff salaries etc won't change much, but it won't be down to $2.80. I personally would assume the same opex, if you want you could drop it 10% or so.