There were massive improvements macrumors posted from the PCIe SSD.
At some point those speed differences become pretty negligible, however.
Going from a traditional platter hard drive to an SSD gives you something like a 300% increase in read/write speeds, it's a massive difference and one that any person off the street will notice in applications opening, transferring files, etc.
You can theoretically about double the read/write speeds from your current SSD with PCIe SSD, but if your data transfers are already practically instant are you really going to see a huge difference if they're more near instant? Enh, maybe.
Is that something worth waiting around months for? Not in my book.
Same goes for wifi speeds...your bottleneck is usually your router, rarely is your wifi card or lack of ethernet connection what's slowing you down. And the video card may be incrementally faster, but the current iMac has some pretty solid video card options available.
There will always be better, faster, etc. out there, but as soon as you buy something the next better, faster thing is going to be in development and coming out soon. any good Mac will last you at least several year and have a nice resale value when you decide to get rid of it, at some point you just have to pull the trigger.