hi,

see there is no disadvantage to study for integrated b.tech+mba course.  generally students appear for GRE/ TOEFEL exam at the final year of study to apply for abroad university for higher study. to be selected in tier 1 college apart from gre score your academic records, letter of recommendation, project or research work publications are also  added scores for your profile. gre score is valid for five years , so you may appear in gre exam at third year.

now coming to gre score there is separate section of logical reasoning and quantitative ,  the GRE total score ranges from 260-340, so 320 + is considered  excellent score.