Upgrading from Sony Vaio vpcz1290x...? Dell XPS13, HP Spectre x360, or??

Hello everyone,

I know this topic has come up in the past but it's been a while and I'm looking to upgrade my 5+year old Sony Z. I absolutely loooove the form factor, size, power (although I'm looking for more), etc. Ideally I would like to keep the size/weight the same or less. I've somewhat narrowed it down to the Dell XPS13, HP Spectre X360, and maybe even a Razer Blade (2015). I love the size of the XPS13 but I'm just concerned about how much of an upgrade that would be. I don't do any gaming and the most power-hungry things I do would be photo editing and want something a with powerful than my Z (i7, 8gb ram, SSD). It seems like I'm already pushing the limits of my Z since it stutters a little in photoshop and sometimes even freezes (this never happens with other programs). All of the "upgrade" computers that I mentioned obviously have much newer processors but the Z also has a discrete video card. And unfortunately the XPS13 and x360 aren't upgradeable to 16gb ram! So would any of those laptops be a considerable step above my current Z (new i7 broadwell processors with integrated video card, 8gb ram vs. i7 M620 with discrete graphics card, 8gb ram in my current Z)? Obviously the Razer would be great (quad core, 16gb ram, discrete graphics) but I would rather have something more lightweight/portable and with a longer battery. What have you upgraded to from your Z and any regrets? Thanks in advance for the input!

PS. Yes, I have done a "What laptop should I buy?" thread but I wanted to get some input directly from people who have owned a Sony Z. Thanks!