OK that's interesting, that's significantly lower probability than a stealer vs. termintor in Space Hulk.
Think the Stealers back then were tougher and didn't need toxin sacks and stuff
I'm not sure what the Stealers rules were back in Rogue Trader and at the time of Space Hulk 1st ed (am pretty sure they were in there, albeit looked quite a lot different). I know in 2nd edition they were absolutely nails (in amongst an army that was all nails!) Off top of my head, WS6, 3 attacks, can't remember if they were S4 or S5, but the way the melee system worked in that edition I remember them charging into squads of marines (or anything actually) and you may as well just remove the squad. A particularly dirty trick was using an allied adeptus psyker and using 'Gate' to just run them straight through and directly into close combat.