Separation of Powers Does Not Preclude a Search of a Congressional Office
Here is an excerpt from US v. Brewster, a 1972 Supreme Court case interpreting the Speech and Debate Clause:
Johnson thus stands as a unanimous holding that a Member of Congress may be prosecuted under a criminal statute provided that the Government's case does not rely on legislative acts or the motivation for legislative acts. A legislative act has consistently been defined as an act generally done in Congress in relation to the business before it. In sum, the Speech or Debate Clause prohibits inquiry only into those things generally said or done in the House or the Senate in the performance of official duties and into the motivation for those acts.
It is well known, of course, that Members of the Congress engage in many activities other than the purely legislative activities protected by the Speech or Debate Clause. These include a wide range of legitimate "errands" performed for constituents, the making of appointments with Government agencies, assistance in securing Government contracts, preparing so-called "news letters" to constituents, news releases, and speeches delivered outside the Congress. The range of these related activities has grown over the years. They are performed in part because they have come to be expected by constituents, and because they are a means of developing continuing support for future elections. Although these are entirely legitimate activities, they are political in nature rather than legislative, in the sense that term has been used by the Court in prior cases. But it has never been seriously contended that these political matters, however appropriate, have the protection afforded by the Speech or Debate Clause. Careful examination of the decided cases reveals that the Court has regarded the protection as reaching only those things "generally done in a [408 U.S. 501, 513] session of the House by one of its members in relation to the business before it," Kilbourn v. Thompson, supra, at 204, or things "said or done by him, as a representative, in the exercise of the functions of that office," Coffin v. Coffin, 4 Mass. 1, 27 (1808).
This quote shows two things:
1.) There is little precedent for arguing that the Speech and Debate Clause protects a place, like Jefferson's congressional office. Rather, it protects activities. More specifically, legislative acts and not political acts.
2.) Any material in the realm of constituent services (which could include evidence relating to Jefferson's attempts to influence Nigerian government officials on behalf of US companies) will be deemed "political" rather than legislative, and will not receive protection.
At most, the Clause gives Jefferson an argument that some of the seized material, i.e. material relating to legislative acts, might be excludable evidence. According to the Brewster case, the Clause does not shield everything that transpires in his office.