I have considered 8"s but for low end extension, I much prefer a 10" (i would go with a 15" if I had the room). Image Dynamics does not produce the IDQ8 anymore IIRC and IMO there are not many other comparable 8s that i would want to go with out spending a lot of money or having one that would require a lot more airspace (W7)...and in those situations you can get equal or better results.

I am curious how you came up with up to 6 dB higher with 2 8s versus a single 10 as i modeled both setups in their proper enclosures with a QTC of 0.71 for best SQ, the airspace required for the pair of 8s was slightly higher than the single 10 as well as the 8s being only 2 dB louder at 30 Hz and at 50 Hz they were equal in SPL and 50-80 Hz the 10 was 4 dB louder than the 8s. This was with identical power and since most 10s are capable of handling much more power than most 8s (most 8s i have seen have a RMS power handing of around 200 watts and that is on the high end) By properly powering the 10 and not under powering it sealed the gap and makes the 10 louder than the pair of 8s at all freq.

My previous install used 8.5" drivers as midbasses. The other problem with the 8s in separate chambers made out of fiberglass is getting the volumes of the separate enclosures as close as possible to prevent one having a slightly different response. As for weight, depending on which pair of 8s, they can easily outweigh a decent 10. The boxes that i am building are not completely MDF and about half is fiberglass so they don’t weigh that much to begin with and the difference between 2 smaller MDF/'glass boxes and one larger one would not really make a difference. The other issue which i have been battling since i started engineering this is being able to reproduce these boxes without using difficult molds or my car over and over. I am experimenting with different templates so that I can reproduce these boxes with pretty good tolerances, but the enclosures that I would need to build 8s in the back, the bottom of the enclosure would need to be a mold taken from the car and doing that repetitive is not easy unless i had a wrecked car to play with.