You raise an interesting point as to the literal interpretation of the phrase "hot as balls."
First, let us consider the use of the word hot. In this case, hot could refer to one of two things, temprerature or attractiveness (slang). If we are considering the temperature of balls, most people know that balls are slightly (1-2 degrees F) less hot than the rest of the body, a result of their hanging away from the warmth of the rest of the body. As a result, balls are relatively less hot than the rest of the body, but still relativel warmer than most things that you would encounter in a typical day. Using this definition, to say that a woman is "hot as balls" would mean that she is slightly less hot temperature-wise than a normal person's body. I am fairly sure that this is not the meaning that you are trying to convey.
However, if you are meaning to then take the measurement of the relative warmth of the object in question (i.e. balls) and use that with the other definition of hot (a measure of attractiveness), you would be saying that the female in question is relatively warm, but not as warm as the rest of the body. A better compliment, therefore, might be to say that the female in question is "hot as my tongue (or other random body part that achives the temperature of 98.6 F)."
The final meaning that you might be trying to convey, would be that the female is as hot (attractive) as balls are (attractive). Since most heterosexual users on ATOT would confirm that balls aren't very attractive (unless referring to their own), it is safe to say that calling a female "hot as balls" might in fact mean that she is not very attractive at all and therefore isn't a very good compliment.
All in all, I think that when using the expression "hot as balls," most users aren't really using it in the context of talking about a female, but instead referring to the temperature. For example, I come from Miami and almost every day during the summer I can walk outside and safely say, "Damn, its hot as balls out."