With the wide blue foam pad in there, it felt pretty similar to no pad. Plenty of room for side sleeping, and still a nice flat lay. I didn't really fiddle with the pad position very much, just unrolled it inside the slot and left it wherever it landed. It seemed to affect the foot box area most, so next time, when I've got more light and more time, I might try moving it more toward the head end.
The cold butt was completely gone with the pad in place, and the only source of any chill at all was when the wind blew through the netting on top. I have no doubt that the tarp will take care of that issue. I'm pretty confident that, with the thermarest stacked on top of the blue foam pad, and a 15 degree mummy bag inside, I will be plenty warm. I will still probably fiddle around with using an old sleeping bag as an underquilt, but it seems like this setup should be good into the mid to high 30's I'm likely to see this weekend. I'll keep yall posted.