Some points here, the PCI-E's are already so fast in rMBP's you're not going see any difference from a performance - they're already topping out at 1800MB/s read and write - thats insane - its 4x the speed of what was considered a super fast SSD 3-4 years ago that saturated (and still does) any Sata bus. You're not going to see any real world difference from 2000, 2500 or 3000 MB/s, other parts are now what slows down the boot speed and unless all you do is duplicate large files all day you're not going to see any difference in file copy time.
Faster and more ram? Again I question what tasks you're doing that could possibly max out even 16gb of ram, nevermind 32. I have 32 in my iMac and use it for actual music composing with multi-gigabye libraries but the SSD is so fast it doesn't even need to load those libraries into ram anymore. You're again, not going to see any real world OS X performance difference with fast ram either - we've plateaued on all these points now. I multi-task like an idiot on my Macbook Pro, I have 45+ tabs open in Safari, plus I have Chrome with 10+ and Firefox open at all times for testing in different browsers, I have Pixelmator, Photoshop, Calcbot, Mail (with 60,000 e-mails in it), Clear, Evernote, Calendar, iTunes, Spotify, Numbers, Tweetbot, Tor, Whatsapp, Skype, Messages, Terminal, Coda 2, Transmit, Deliveries and a hanful of other apps open all the time - and i'm using barely more than a quarter of the 16gb of ram available on my rMBP.
You talk about gimmicks and then mention 4k screens. You realise its barely possible for the eye to tell the difference on a 50" screen from normal viewing distances with 4k, let alone a 15" screen. We're already at imperceivable retina resolution - the resolution does NOT need to go up anymore on a 15" Macbook - we need to deal with colour accuracy, gamut, OLED screens (that need to come a long way yet before they improve on LCD's outright), linear all over performance, power efficiency - resolution is the one thing that doesn't need upgrading and if they did that really would be a pointless gimmick that just saps power from your battery cycles unnecessarily and wastes GPU performance for no added benefit to the end users - hell most of the internet still hasn't even caught up with retina resolutions yet (as I look at my blurry profile picture on MacRumors for instance)
While I can't say i'm too bothered by the two aforementioned features of Touch ID and and a OLED touch strip (although we've not seen how well they might be implemented yet) its things like this that will pull in more users that upping any of the above i've mentioned that have now reached a point that wont make a jot of difference to the end users daily working. If anything its the raw processor power thats lagging behind now - the PCI-E flash loads faster than the CPU can process the OS booting (not that I ever shutdown and cold boot mind)