Orly: You are correct about per-pixel sensitivity. I tried to be clear that with double the Q.E., the per-unit-area sensitivity doubles. I used that term to differentiate from per-pixel sensitivity. For one, I don't think pixel sizes have quite halved. The 450D had, 5.1 micron pixels (? don't have mine handy), the 7D II has 4.1 micron pixels. That's an area factor of 0.65x. In terms of per-pixel sensitivity alone, the 7D II gains at 59% Q.E., and loses at 0.65x the area. That gives the 7D II a 27% advantage in overall light gathering capacity. That only factors in the increase in Q.E., though. There is also the reduction in read noise. Even better, there is the MASSIVE reduction in dark current (the 7D II has one of the lowest dark current rates of any digital camera on the market, apparently even 10x lower than the 6D, which was already pretty low.) The higher Q.E., reduced RN, reduced DC, all lead to a significantly higher SNR per-sub out of the camera.
Regarding the NX1, body only is $1500. Not a bad price, but certainly more than a used older Canon DSLR. I've got some data from an NX1. Personally, I think it's VASTLY superior to any Canon data, particularly at low ISO. At higher ISO the gap narrows, but I prefer the very neutral kind of data strait out of the camera. The 7D II still has that fairly heavy red shifted data in their RAWs, and it STILL has banding (even though it's lower). I saw no hint of banding in the NX1, and minimal hot pixels.
I don't know if anyone has used one for astro yet. At the moment there is not an SDK, however Samsung seems to be fairly on the ball with firmware updates, and the SDK is supposed to be coming soon. It's supposed to be a very rich SDK, I gather that Samsung is taking direct aim at Canon's 7D line of cameras (and the specs of the NX1 seem to back that up...I'm quite impressed with it specs wise, and can't wait to rent one and give it a whirl for birds, wildlife and astro.) I have very high hopes for the NX1, however personally, I feel it's just a little too soon. I don't know that Guylain would be interested in investing time and resources in something like BackyardSamsung any time soon, not with less than a handful of people using any of those cameras for astro. I do think Samsung is a solid new player, and if they keep up their current momentum (which actually includes a 300mm f/2.8 lens that seems to rival Canon's own prime superteles, again at least specs wise), I think they could be a very, very solid contender for top Astro APS-C camera.
If you are really interested in it, I would check out LensRentals.com. I already verified that they have NX1's in stock for rent, and the cost is pretty decent. I plan to try one out myself, along with a Canon EF adapter and maybe one of their longer zoom lenses, once bird migration season starts again. It's the top camera on my list at the moment...it really, REALLY intrigues me.
Regarding full frame...I think you could use more than just the FSQ106 with it. Any of the Edges have a flat 44mm field, and many of the AstroTech RC's do as well (particularly the larger ones). Personally I use my 600mm f/4 canon lens with my 5D III, although at that price your better off with the FSQ. I don't know of any of those other scopes are going to have as flat and unvignetted field as the FSQ106...it's base field is 88mm, so the 44mm center field should be pristine. I don't think I can even say that about my 600mm f/4....
Anyway, for APS-C cameras, personally I think the top dogs are the NX1 or the 7D II. The former is actually even cheaper than the latter, but it's not going to have much in the way of software compatibility. (Also, at the moment...I think the current firmware only supports 30-second bulb, but given how programmable the camera is, and given how fast Samsung already came out with firmware 1.2, it may not remain that way for long.)