I'm not opposed to doing both 10 and 25 man stuff. Heck, we may be able to do the 25-man content with 10. I'm not exactly sure of the scaling between 80 and 85 for raiding.
As far as the day goes.... I'm figuring a Tuesday or Thursday. Just about every other say is occupied by DB raids and Sun/Mon is right out for me.
I would think Thursdays would be ideal. No raids scheduled and we tend to have a lot of people on. Tuesdays are typically taken up by people trying to get in a BH run and then a quick heroic before logging off.
To give you a sense of scaling between 10 and 25 (for Lich King). Cad was doing just shy of 30k dps, and was able to tank LK for almost 2 minutes straight. (in dps spec).
My expectation is that with 10 dps and 2 healers we could do 25 man LK content provided execution was solid.
Ulduar 25 would be a walk in the park for a 10 man raid.
Either way, Avrus has 4 mounts to go for his 100 mount achieve and I am missing drakes from Ulduar 25, LK 10 & 25 and the Cataclysm heroic dungeon achieves.