I've decided to take the plunge and order the Audison AP8.9 bit. Doing some work with Marty from FHRX, we've estasblished that the front doors have their own crossovers in them, so a separate channel isn't required for each door speaker (given there are four speakers per front door, I'd either have to run a 4-channel amp per door, or introduce a crossover in each door myself).
I'll be bridging two channels for each front door, run each back door off a single channel, and will bridge two channels for the sub. I'm not going to bother with the centre speaker at all - from what Marty has said, the McIntosh headunit/amp combo just synthesises what belongs in a centre channel anyway (given there's no centre channel information provided by any media my headunit can play. Worst case, if we find I do need the centre channel, we swap the centre speaker for the sub, and I grab the AP1D to drive the sub).
the AP8.9 bit isn't just an amp either - it also has its own DSP built in, and it was designed by Audison for OEM integration to start with.
In addition to the amp, I'm also grabbing two boxes of Focal BAM XXL (each box has a roll 50cm wide and 3m long, so 6 square metres). I'll be focusing on front doors and parcel shelf to start with, and then other areas of the car as Marty suggests (he's already mentioned wheel arches). The BAM does two things - dampens vibrations, but it also cuts noise coming into the car (Marty mentioned as much as a 6dB decrease in outside noise, depending how much of the stuff you use). Each box weighs less than 3kgs, so the weight it adds to the car isn't significant. With the BAM and a decent amp (we discussed some options other than the Audison, but this seems the best option at this stage), Marty said the stock speakers actually do quite well.
We'll see how things sit once it's all installed.
