All right...there is a lot of "magic" when it comes to powders, so here's what I know, based a lot on what some experts know. I'll try to keep clear on what's just one man's opinion.
From very reliable, published sources: All the double-base Reloader-line powders, except Rel-17, are made by Bofors. What Bofor's makes & sells would be considered "bulk" powder, and has to be within 20% of the established burn rate to be called a certain powder. The repackagers, Norma and Rottweil in Europe, and Alliant in the States, are selling what is termed "cannister" powders, and they must be within a 10% variation in burn rate to get the designation of, say, Reloader-15. (See page 574 of Ken Howells book, Designing and Forming Custom Cartirdges. Also German Salazar.
Now, Alliant -- Reloader -- has historically used all the 10% allowed variation. Back around 2000, there was a lot of Rel-22 that got out which was more than 10% faster (acknowledged by Alliant), and Bobby Hart (they sell a lot of the stuff) reported that most of the line seemed to be about one grade faster that year.
In short, it happens.
Here's point 2: While the basic powder is double-based, the size of the granule is still of some importance for burn rate. Historically, Alliant has mixed granule sizes to achieve the needed "10% only variation" for cannister grade powders. I get a laugh when some long-range shooter says they weight to the nearest granule -- but which one? Withe the Reloader line, sometimes, the smallest granules are half the size & weight of the largest.
So be careful with that sifting, if you filter out the smaller granules of "Reloader 15," along with the ball powder, you may wind up with Reloader 16 -- or slower.
Also be aware the military pull-down powder was "bulk," not "cannister." Though for military sale, it would have been loaded to a mil-spec burn rate, we just don't know what the appropriate charge weight for that powder was.
So I don't see any real need to sift out the ball powder. It's just one more variable. I do see a need to start 20% low for one round, working up and watching for signs of pressure. After resolving any pressure issues, it may turn out to be the most accurate powder you've ever had, and you'll go crazy trying to get more. Or not, who can say?
Finally, both double-base and ball powders have the reputation of being temperature sensitive. Well, all powders are, but enough people have remarked on this to keep it in mind. In my experience (and Jeff Summers), the original Rel-10 worked great if it was significantly above 80 degrees. Poorly below that, though. And dirty in either. So in your testing, watch the outside temperature. Don't relay on pressure observations if there is a 20-degree difference. And if it shoots crappy when it's cold, try a few rounds in the summer's heat...