Are you really trying to say that Space Marines are better in CC than genestealers?
Space Marines shoot genestealers to death, and genestealers slaughter Space Marines in combat. If a Space Marines kills a genestealer, it's because of his bolter, probably not because of his bolt pistol.
Oh, wait, oops. Yeah, you hit T2 on a 4+ with re-rolls. My bad. So you hit T4 and below 75% of the the time, and T5+ 50% of the time.