Don't forget, Sisters of Battle are ( to my knowledge anyway ) not actually genetically engineered superhumans. They're just humans in really big armor, with better training and firm conviction. And they're still Toughness 4, if I'm not reading off the wrong unit. So they are better!
Also, 3 BS vs. 4 BS may not sound like a huge difference, but it's a pretty big difference in-game ( very few things go above 5 ). Also, they have stronger weapons, meaning the average sister will be killing a lot more things than the average guardsman. Now, you could probably take something like 4 guardsmen for every 1 sister, and they may not be perfectly balanced - that depends on the size of the game in general, as well as other factors.
Tau are sort of similar. A Fire Warrior has only BS3, which is weird for a race dedicated to shooting things. It's basically a balance thing in their case, because they can offset this by using markerlights - and their weapons are absolutely devastating, at S5. Which basically means that while their pretty lousy shots, they're much more likely to kill things if they hit.