Hardware advice needed

So I'm going to invest in some new hardware. I'm starting with a new iMac. We're currently running on mid-2011 i7 iMacs with 16GB ram and SSDs. They're generally quite speedy and have served their purpose well. But we're now editing the most demanding project to date for us and it's bringing the machines to their knees. And we're not even editing 4K!

For this project, it's not render speed that's the issue, it's general interface performance or "snappiness"/responvieness. It's like water torture. I'm not exaggerating when I say I spend more time waiting than editing. Lots of spinning beachball and just waiting for the computer to catch up with me. And I don't mean a half second or so, I mean several seconds for each and every action. Excruciating.

Here's what I have found that really slows things down:
Use of auditions
Izotope's RX4 audio plugins
Surround sound pan modes

And I've listed them in order of pain. I've saved audio sweetening for the end. Technically, I could finalize the auditions, which would help a bit, but I just hate to do that. I could process all the audio externally, but sheesh, it's so convenient to do it right in FCPX. For our project, we don't have the budget or skills to do a proper Pro-Tools type mix. And surround sound, my god what a resource hog. I had no idea. And proxies don't seem to help in this case (since it's the audio stuff that's really crushing the machine).

I'm willing to spend whatever it takes to get back to having a no-lag editing experience. I don't mind waiting for renders, but I really super hate having to wait for the computer to keep up with my trims and fades and pans etc.

So it boils down to this. In your collective estimation, would the new maxed out Retina iMac get me closer than a maxed out non-Retina iMac? On paper it seems the answer is yes. But I'm a little unclear as to how the GPUs would affect general responsiveness. Since each uses different GPUs, I figured that might make a difference. I don't particularly need 5k, I just need super snappy responsive editing. When I click and drag, the damned cursor better move with me.

And that leads in to the second part of my question. Again, in your collective estimation, how much "snappier" would a maxed out 8-core nMP be compared to either maxed out iMac? I don't mind investing in such a beast, but only if it's noticeably "snappier" than other options. And to be clear, when I say "snappy", I mean when you've got three Izotope filters and surround sound on an Audition clip set, I expect there to be no lag whatsoever when I start editing that clip. Right now I literally wait several seconds. Pressing spacebar to play such a timeline can be a 15+ second affair. Agony.

Hackintosh is not an option for me. Not interested.

What say you wise men and women?

(Side note; A feature I'd love FCPX to have and that I've submitted to Apple is to have a simple toggle button that basically turns off EVERY filter and effect so you can play your timeline without worry about it choking. Right now it's too hard to go in and turn off stuff manually for performance.)

The 2011 iMac 27 CPU varied from a 2.7Ghz i5 up to a 3.4Ghz i7 ("Sandy Bridge"). The GPU was either a 512MB Radeon HD 6770M or a 1 or 2GB Radeon HD 6970M. The SSD was SATA-based, not PCIe.

So depending on which 2011 iMac config you have, from a pure hardware standpoint a top-spec retina iMac would range from "considerably faster" to "vastly faster".

That said I'm not sure hardware is the solution for the magnitude of problem you're seeing. E.g, a 15 sec. delay to start playback, etc. Even IF a top-spec retina iMac was 300% faster, you'd still be waiting 5 sec, which is way too long. Even a nMP can't fix that. Something else is probably wrong.

Regardless, given your stated workflow and need for editing responsiveness, I think a new top-spec iMac is probably a good idea. However I don't think it will fix your problem. I suggest you re-post the essentials of this on the FCP forum, phrased as a general performance problem, and ask what troubleshooting steps within FCP X are prudent.