I do not have a Dell XPS 15... and I am not sure when the 2018 Dell XPS 15 is expected to be released.
What I can tell you is that the new 2018 version can have up to the 6 core i9-9950HK. It is clocked around 500MHz faster than the 6 core i7-8850H. It also has a 33.33% more internal L3 cache compared to the i7. Without getting too technical that is basically very fast RAM built inside the CPU that can help improve performance a little bit when the CPU needs to make very complex calculations. Example would be video encoding and Monte Carlo financial modelling simulations. I assume most people would opt for the i5-8300H CPU which is basically like the "old core i7 CPUs". Both have Hyper Threading (HT) which means each core can process 2 sets of instructions instead of only 1 set of instructions.
Generally speaking, 7th gen Core i5 CPUs are only quad core CPUs. The 8th Core i5 CPUs are quad core CPUs, but they also have HT... Just like the 7th gen i7-7700HQ. The vast majority of games do not take advantage of HT. Overwatch and Tom Clancy's Rainbow Six are 2 of 3 games I am aware of that can take advantage of HT. I cannot remember the 3rd game.
The new 2018 Dell XPS 15 will have the nVidia GTX 1050 Ti which offers better game performance compared to the GTX 1050 in the current Dell XPS 15. Also, the vast majority of games use 4 cores or less... Buying a laptop with an 8th gen Core i7 or Core i9 with 6 cores (and Hyper Threading) may sound awesome, but if what you do cannot make much or any use of the 5th and 6th cores, then you are basically spending extra money for nothing. I am sure there are a handful of games that can make use of more than 4 cores and HT, but I do not know of any other than Overwatch to a minor extent.