Definitely the newer laptops/CPUS are going lower power (consumption and processing) My 3.5 year old XPS 15z with a Sandy Bridge i7-2640M still keeps up with most of the current smaller laptops (I've been looking for a 16GB Ultrabook size machine)

As mentioned you are missing built in Ethernet on that XPS13. I do like the look of them though. Fixable with a USB dongle but could be annoying.

I don't think you'll notice the performance hit all that much unless you are doing a lot of CPU intensive work like video encoding or similar.

Why do you have 24GB of RAM in your desktop? That's quite a lot. I do have 16GB in my XPS 15z but that's because I run a couple of VMs typically (e.g. a cross compile environment) and I generally like keeping a lot of things open. 8GB tended to run out a bit after giving ~4GB or more to VMs.

If you google "<CPU A> vs <CPU B>" you'll get hits comparing them together which I often find helpful e.g.
http://www.cpu-world.com/Compare/88/Intel_Core_i5_Mobile_i5-5200U_vs_Intel_Core_i7_i7-920.html
http://cpuboss.com/cpus/Intel-Core-i7-920-vs-Intel-Core-i5-5200U

I note single core performance is a bit better in the latter link. Many things still only run single core remember and I suspect your Passmark links are factoring in cores so your 8 (thread) core vs 4 (thread) core is the difference putting them on a par for a single core.

Video will be no problem and basic editing the same. Maybe in the odd situation your old desktop would be faster but personally I doubt you'll really notice based on what you've said.