I dunno. In the Senate, she'd be just one in a group of one hundred, but as SoS, she's in a solo position of great (perceived) authority, and very high profile.
I think she can do much more damage to Obama from her current position than from the Senate, by differing with him on foreign policy, and her relations with foreign heads of state.
An interesting article;
Hillary Clinton Grinding Away In Obama's Shadow.
Some snippets;
The challenge is one that nearly all Cabinet officers face in the modern presidency. It is that a grand title -- and in Clinton's case, a fancy blue-and-white Air Force plane at her disposal -- does not automatically translate to policy influence. Such outsize personalities as Alexander Haig and Colin Powell have found themselves at State essentially playing with a toy phone.
George Stephanopoulos pressed her to describe "What is your role, exactly?" She answered vaguely about being "chief diplomat" and eventually answered, "The president asked me to lead the effort on food security."
"Between her consideration and her final confirmation she had lost some authority and power as all of these envoys were appointed," "Once she did get confirmed, though, what we have seen is a steady increase in her authority and control as we have seen envoys seeming to now work with her."