The two components that draws the most power, thus producing the most heat, are the CPU and GPU. 7th generation Intel mobile core i5 / i7 CPUs are rated at 45w TDP. That basically means they produce 45w of equivalent heat, which is different from actual power consumption. A core i7 would generally consume more power than a core i5 CPU. The basic difference between the i5 and i7 is that the i7 has Hyper Threading (HT) which basically means each CPU core and process two different streams (threads) of data. The vast majority of games do not really get better performance with HT capable CPUs. The one exception in an extremely small handful of games that do is Overwatch.
Dropping down from a GTX 1080 to a GTX 1070 should also help with lowering the heat since the GTX 1070 should use a little less power. But the only way to be sure if that is actually the case is to test two Alienware 17 R4 with a GTX 1070 and GTX 1080 to see if there is an actual difference.
As for screen resolution, from what I know the Alienware can be configured with either a 1920x1080 IPS screen or a 4k resolution TN screen. The IPS screen will have better colors and viewing angles, but like all IPS screen it only has a 60Hz refresh rate which means at most only 60 FPS can be displayed on the screen even if the GTX 1070 / GTX 1080 can render 120 FPS in a particular game. The TN screen allows you to play games at up to 4k resolution as long as the game can support that resolution. However, that is 4x the resolution of 1080p; the higher the resolution the lower the performance. I don't know if the TN screen is 60Hz or 120Hz. While the GTX 1070 / GTX 1080 is capable of playing many games at 4k resolution, the performance will definitely be less than 60 FPS.