As for the speed, at 15meg, that would be enough for what you are doing, but if you find it slowing down, then again it goes back to what the wiring is like, and with CATV ISPs, you are sharing your Internet connection on one large LAN, which means if you have users that are abusers and congest the LAN with downloads, it slows everyone else down. Where as with ATT, you have your own dedicated line back to the VRAD, which goes back to the ATM.
If you are actually 600 to 700 feet from the connection point, you would see probably anywhere from 55meg to maybe 65 meg for your Max User rate, your User rate would be 32/5. The difference between Max User Rate & User Rate, is what determines what will happen inside from the RG to your devices & computers that are connected to it.
I see as low as maybe 17mbps with all three tv's on, my wife on her iPad watching videos, my son playing games on his PS3 with others, on his laptop listening to music from the net, and I on my netbook browsing news sites, etc, which do not use a lot of bandwidth. If I am watching Netflix, and we have the other two tv's, and the before mentioned, we still do not see any lower than 17mbs.
Now this announcement came about within the past week New Speeds & equipment coming to UVerse