That spec iMac is currently $3879 at MacMall [1],
To save money, you could easily spec a 4.0GHz/8GB/1TB FUS/M390 for $2299 from B&H [3], add the same SSD ($300) and ram from OWC ($328 for 32GB / $667 for 64GB). Bringing the total to $3266. We also need to go to a 5K 27 inch monitor on the PC side for a fair price comparison; the cheapest I could find was $1042 HP Z27q [4]. Add $42 to the PC price to make up for this.
Now, the PC price comes out to $4370+42=$4412, so we need to drop $1146 of from the PC to get a realistic price comparison. The RAM, Screen, SSD, and graphics card are anchored due to our Apples to Not Apples comparison. That leaves the motherboard, case, CPU, power supply and water cooler to drop in price.
I don't know water cooling particularly well, but lets say we leave that in to continue overclocking the CPU, but drop to a lesser model, same with the Power Supply and Case. So revised budget for each is:
Case: $125 -> $80
Power Supply: $140 -> $80
Water cooling: $120 -> $80
Now a gaming motherboard was selected because it's stable for overclocking, but let's throw caution to the wind and drop our budget to half that and hope it still overclocks stable. $480 -> $240
That leaves $261 for the CPU, which buys an i5-6600K (3.5GHz), or we drop the water cooling and run everything at stock to get to an i7-6700K (4GHz). In other words, the same processor in the iMac. If this benchmarks any differently, it's the Lightroom developers fault, not the hardware.
So, the upshot is, spending the same amount on a Mac will get you the same specs in a much better looking equivalent device.
[1] http://www.macmall.com/p/product~dpno~13697228~pdp.jhffibb?s...
[2] http://www.bhphotovideo.com/c/product/1190421-REG/apple_z0sd...
[3] http://www.bhphotovideo.com/c/product/1190421-REG/apple_z0sd...
[4] http://www.bhphotovideo.com/bnh/controller/home?O=&sku=11429...