Memory makes a big difference. Definitely get the 16 gig.
High-end CPUs are best. Rumor has it that 4 cores are best for LR due to performance problems with 8(?).
The GPU may or may not help. If you don't have a 4K monitor or bigger, you probably will not see any advantage gained.
Agreed. The 16 gig is sort of minimal for Lightroom work. Certainly with 42 MP files. Get as fast and multicored a CPU as you can get with this machine. Lots of stuff is threaded in Lightroom and since Lightroom does well with up to 4 logical cores you don't have to worry about having too many. People with 8 cores often report very slow behavior. The discrete GPU will help quite a bit on this machine even when you don't use an external 4k or 5k display as it has a retina display itself. On such machines GPU acceleration really makes a big difference. The GPU on the 13" will also accelerate operations in Lightroom though not as well as the discrete one in the 15". So unfortunately the answer is to get it all. If I could choose only one thing, most important is to get the 16 gigs and shared second place is i7 and discrete GPU.
CPU speed is always important in Lightroom. For your very large original files, I would get the fastest CPU you can afford, but I have read the reports that more than 4 cores sometimes doesn't help and perhaps will slow you down. Adding memory will provide a secondary speed effect compared to the CPU.The GPU will speed up certain actions in the Develop Module only, if the GPU meets the specifications (and most newer GPUs do). If I could choose only one thing, it would be to get the fastest CPU you can afford.
Catalog size has NO effect on Lightroom speed (with one exception, the speed of making a backup of your catalog file, the larger the catalog, the longer the backup takes).