I won't pass judgment on what is a tough target or what is not a tough target.
Recent research that has looked at this question of what total cumulative emissions are consistent with a limit of 2°, or a limit of whatever you would choose to set—that again is a policy question, not a scientific question—indicates that the choice of baseline is actually not that important. So a 1990 baseline allows you to construct emissions pathways that would get you to 2°C with reasonable likelihood. A 2006 baseline also lets you construct pathways that allow you to get to 2°C with reasonable likelihood.
All emissions scenarios are constrained by the emissions that we have already produced and the fact that we cannot go back in time and reduce emissions or the pathway we have been on in the past. That is really what constrains what these pathways would look like. All of them would require peaking relatively soon and then substantial reductions in greenhouse gas concentrations and emissions in the atmosphere.