...I'd be looking very heavily at customizing the impulse response technology, which was never designed to emulate cabinets...
Ermz, you're a good mix engineer, and generally a good resource of information on this forum, but I'm going to have to bust you on this one, in case anyone makes the mistake of taking it as gospel (as you have a habit of being right most of the time.)
Impulse response technology was never "designed" to do anything with audio whatsoever - convolution and most of the equations used were developed in the 1800s, in fact. There are a lot of applications for convolution, far outside of audio DSP, including graphics, RADAR and other defense related technology, etc.
What you're saying is akin to saying "multiplication was never designed to adjust volume" - however, when you mix in the box, that's exactly what your computer is doing. Your computer has never heard audio in its life - it's happily crunching numbers for you all day, period.
Convolution and related algorithms (Volterra kernels, FFT, DFT, etc.) are mathematical tools that in the right hands can be used to create a near endless variety of different kinds of software - and these equations existed long before computers.
Right now, in the digital audio space, there is a stupid pissing match happening between so-called "black box" techniques (convolution, Volterra, etc.) and "white box" techniques (circuit-based modeling) for emulating the sounds of analog equipment. Ironically, nothing in digital is truly "white box" - nobody is emulating things at the molecular or atomic level (that I know of) for example, so these terms are extremely relative. Even circuit-based models make the assumptions that certain algorithmic processes adequately emulate various electronic components found in analog audio gear, for example.
There is nothing intrinsically superior or inferior about white box or black box modeling - both can be great or terrible - it really depends on the specific implementation and the needs of its audience. Also, both approaches can be (and often are) blended into so-called "gray box" solutions.
Back to your point about "customizing" impulse response technology itself, there's a variety of possible approaches to nonlinear convolution and other related technology, and all of them add up to very high CPU usage at the moment, which is the primary reason why you don't see more solutions based on this kind of approach. Nebula 3 is (at the moment) the bleeding edge of black box, and is very impressive at a few things - a hint of some of the stuff you'll begin to see later - and I'm sure it will continue to improve along with other solutions.
As has been noted earlier in the thread, SSE2 has made a lot of things better for audio DSP, and the next 2-5 years should see the rise of some more powerful, exciting, and dramatic FX plugins.
Sorry for the OT - I'm looking forward to the Fractal announcement.
