Coopers Extra Strong "13

The Coopers site seem to think it could come out as a high carbed beer

STEP 3: Bottle
The brew may be bottled only after the SG readings are stable over a couple of days – it should finish around the 1008 to 1012 mark.
We recommend the use of PET bottles. Sturdy reusable glass bottles, designed for storing beer, should be used if planning to keep some of the brew in bottle beyond 18 months.
Prime at the normal rate of 8g/l (2 carbonation drops per 740/750 ml bottle) or slightly lower, in accordance with the style.

but the half filled bottles would be over carbed as thats a lot of sugar for a little bit of beer.

I find most strong ales to be weird tasting and usually take a long time to condition at least few months, by that time the hops will of mellowed a lot and the malt will taste better too. I think its a challenging beer flavour wise if you arent used to them, not sure if you are?

Give them some more time and see how they go!

Strong beers take a longer time to carb too