I love the imacs, i do own 2 of them 27". Later 2013 imac highest nvidia gpu, cpu, mem spec configuration as well as a later 2014 retina 5k, also highest amd and cpu/mem spec configuration. I would say imacs are not up to the task of running extreme apps like xplane simulation.
Imacs are a beauty, sleek design, quieter, no cabling all around, awesome screens, osx is just great to me. They are all what you need for the conventional day to day home/business tasks.
But for extreme gaming/simulation? I would say they are no go based on my experience so far, you simply have to make too much compromises. They are severely restricted in wattage, they run the -mobile- hardware version of their pc counterparts but they cost 3 times than a pc.
I will be selling one of them soon (i will keep the other for my day by day) and for that money i am able to get one of the most highest spec powerfull available pcs today including newest gtx 1080 from nvidia that will outperform the imac 2 times easily for 1/3 of the original imac cost higher spec configuration.
just my 2 cents.