First, you need to know what pressure level the factory ammunition is loaded to. In other words, is it really +P, or just marked that way! You need to know what +P standard is being referred to. I know that seems inane, but you will find figures from 17,500 PSI to 20,000 PSI published in various loading manuals and reference sources. Likewise you will find standard pressure shown anywhere from 15,000 PSI to 17,000 PSI.
Assuming that whatever load you want to compare is really a +P load, then you need to know what velocity the ammunition really delivers in a specific revolver/pistol. That takes a chronograph. Then you need to load and shoot the published load recipe you are curious about. This has to be in the same gun, same conditions. Preferably same day and fired consecutively with the factory load.
Can factory +P loads be duplicated? Yes. For .38 Special Unique, 4756, WSF, Universal, Herco, HS-6, and several others will safely equal +P performance with lead bullets, but not necessarily directly using data from the published manuals.
So far as you original question is concerned, who knows! Some published reloading data may very well surpass "factory" loads. This is just too open ended a question to answer any more completely than, again, who knows!!
You want to know, buy a chronograph.
FWIW, Federal and Winchester 158 LSWCHP +P have both chronographed at 821 FPS from 2 Model 12s I own, Federal from one, Winchester from the other. The same load ran 945/936 FPS from my 6" 10-4 on the same day. These were relatively small samples in all cases, only 6 rounds for each string. Larger samples will give a more statistically significant result, but these results should be in very close proximity to results with larger samples