To take advantage of AE's ray traced rendering you must have an NVIDIA card. The current line of NVIDIA graphics options from Apple are supported. I'd suggest a MacBookPro Retina if you need maximum portability. You could also use an iMac and carry that around in an iLugger case. I carried an iMac around for several years because it impressed my clients a lot and made closing deals easier.
If you're tight on funds and your credit is pretty good Apple has great options. Just make sure that you've got an approved graphics card, and get yourself a keyboard cover. It's saved my computer many times. They are cheap insurance.
However I worry I'll make a poorly judged purchase if I dont know whether the Retina will actually deliver my production needs... I require speed to maintain a steady flow of projects (and completion)...
Do you know of any systems out there that could be roughly the same price but more optimised for video editing / 3D motion graphics?
If you want to stick with Mac then the Retina Display MacBook Pro is the most powerful laptop you can buy. If you want real speed then you'll need a desktop and count on spending a lot more money to get a desktop. A good production desktop that's really going to have some significant speed for doing production work is going to cost you $6K or more. It's not uncommon to spend close to $10 for a studio grade machine.
My MBPro R is the fully loaded one with max ram and max storage. It works amazingly well for a laptop. The Retina display is marvelous to work on. There's no magic bullet here. Build your own machine from scratch and deal with that basket of problems while saving some money or buy a windows machine from a reputable manufacturer or a Mac Pro and deck it out and you'll be spending about the same for the same components.
If you get really swamped then you should be able to afford another machine for rendering. There's also always rendering services.
If you were working on an older MacBook Pro any new MBPro is going to be significantly faster. Portable and maximum speed are not compatible.
If you're serious about using OSX and not Windows, you'd be crazy to buy a portable machine that isn't built by Apple. Hackintosh systems are flaky at the best of times. The last thing you want, if you're a professional, is to have your machine start misbehaving in front of a client because you're running an unsupported OS on hardware that was never intended to run it.
The reality is that you can probably buy a Windows portable that slightly outperforms a Mac portable, althought the price difference will be minimal at that end of the high performance market. For my money, the MacBook Pro retina display machines are great value. I prefer OS X, which the machines are built to run. I can connect my MBPr to 2 x Dell 27" hi res displays, plus an HDMI preview monitor, and render an AE project in Adobe Media Encoder while working on another project in AE.
Apple's current raft of top-end machines are just that - great work horse computers with excellent performance. Get the highest specs you can afford. If you can't afford one, forget OS X and shop around for the highest spec'd Windows machine you can afford and use it with Windows.