Dead or Alive 5 pushes everything a level above Tekken Tag 2, even matches character count without dropping res. The difference is that the engine they built was optomized for PS3 and 360, whereas TTT2 is looking like a downport from arcade hardware, it has little to do with console limitations and more with developers not getting enough time to work on their engines. For me the loss of background objects is going to be really jarring in some levels.
All graphics are not the same under the hood. Dead or Alive 5 has a different art style in comparison to Tekken Tag 2. Tekken also apparently has more animation data in play, I'm under the impression it's more on the level of Virtua Fighter there.
And the issue with losing background objects has everything to do with CPU limitations on the current consoles. Developers have been bluntly talking about that for a while now, as to why there is not a magical fix for net code on the current hardware.
Street Fighter X Tekken is barely able to run without stuttering because of the complexity of the visuals and stage animations, while attempting actual rollback net code in a fighter with 3D characters. Soul Calibur V doesn't attempt to render 3 - 4 characters on screen. VF5:FS has visuals that from a technical standpoint, don't push the current consoles at all.
The priority in TTT2 was stable and smooth netplay, as in a critical priority. We were warned multiple times this was extremely difficult due to Tekken's data structure specifically. If the early reports about the net code performing very well are true, then they probably made the correct sacrifices.
As for downports, the arcade hardware the game was built for is PS3 based but is reported as upgraded further from the version Tekken 6:BR runs on. Tekken 6 / TTT2 also use features that SCV and the other games don't all have, such as dynamic sand/snow/water deformation around characters and high quality motion blur on character models.
With all these rigors combined together, it sounds pretty realistic to expect walls to be hit when porting down to the current HD consoles.