From the stuff I've had the chance to dyno, and seeing them put into various cars with differing levels of "race effort" into the overall package, I look at the dyno results and feel if the car is on the "good" end of the spectrum, the numbers will be pretty close to the Moroso scale. For anything that is leaning more towards strip than street, you should be well within a 10% margin. For cars that are really more street oriented and seldom see track time, the numbers can (and often are) way off.
Also, obviously, if you race in an area where the weather conditions aren't conducive to making power(like Denver for example), that needs to be factored into it.