In terms of power, Reshiram and Zekrom are entirely on par with previous legendaries. Zekrom's Attack matches Groudon's and Rayquaza's, Reshiram's SpA is just shy of Mewtwo's and matches that of Kyogre, Rayquaza, Dialga, and Palkia. Reshiram's STAB is resisted by only one Pokemon, Heatran.
A couple of interesting points. High-BST legendaries in general have only moderate speed. Mewtwo, Arceus, Lugia, and Palkia have base 130, 120, 110, and 100 respectively. Rayquaza and Kyurem sit at 95. The others all sit at 90, slower than a lot of the 600 and 580 BST legendaries, and a lot of other Pokemon. I suspect this is intentional by Game Freak, in order to somewhat balance out the extreme stats.
Also, there's a clear similarity between the 3rd and 5th gen cover legendaries. Groudon and Zekrom are physically based, with the same HP, Attack and Speed. Kyogre and Reshiram are specially based, with the same HP, SpA and Speed. Rayquaza and Kyurem are both balanced, with the same speed that's 5 faster than the others in their trios.