While we're here, a quick example of four bone stock Z's making 4th gear pulls. Two are 7AT, 2 are 6MT. Variance when torque and horsepower are equal is about 3-5, and peak values on all four are around 275/225, give or take a few by that margin.
That variance of a few whp/wtq is probably due as much to ambient conditions (note the heat and/or humidity as well as air pressure) as anything, but other transient factors are probably at play too (e.g., wheelspin, break-in success, general measurement error).
Also note that some of the SAE correction factor (CF) values are pretty high -- as a rule of thumb, any CF > 1.02 is probably overcorrecting by a bit.
However, also note that STD CF's tend to be HIGHER than SAE, meaning they tend to be REALLY optimistic (I sometimes call them the "bragging rights" CF -- legitimate, but skews high rather than low). I personally prefer conservative over ego-sparing estimates. Think of SAE values as "sleeper" numbers -- your car is probably a bit more powerful than this number on the road; with STD, it's probably a bit less so in real life.
But that's for NA cars. For FI, the higher STD value is probably more accurate, because no CF takes into account the extra heat of compressed air, meaning SAE tends to greatly undercorrect by comparison.