Would not recommend using TOEFL passages as representative practice for GMAT RC. TOEFL passages typically tend to be much simpler, and comprehension is not tested at near the level at which it is for the GMAT.
The only case in which practicing TOEFL passages might help is one where the test taker has almost no prior experience of dealing with RC passages at all and is looking at basic 'primer' essays to get started. Even then, this should be used only as an initiation, and the test taker should quickly try to graduate to short GMAT passages to get better practice and be exposed to principles and concepts specific to the GMAT.
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