Yeah you’re very likely right about the +10% elite being flat +10% in that case. 
But like I said, I don’t feel like that ultimately matters, the 10% is just such so little. Afaik purge & Soulblaze both always hit the “base type” of the enemy, so hit location or armor in head or torso (like Reapers) doesn’t matter. Ofc enemies covered in armor treat that as their base type, so Maulers are flak, Crushers carapace, etc. I haven’t actually tested exactly which of the flak enemies in my google sheet pic above count as flak for their base type, but at a glance we can roughly estimate something like this:
- +25% Flak for purge & Soulblaze probably covers at least around ~30%+ of horde, 1 special, ~30-40%+ of elites, plus captains & probably Karnaks
- +10% Elite covers all elites, period
Assuming all elites were equally likely to spawn, then if +25% flak counts for at least 4/10 or 40% elites it would equal +10% elite overall in that category, and be better in others. If non-oggies are more likely to spawn than oggies (idk if this is the case but it feels like it might be), that would work for flak’s favor considering it’s more weighted to non-oggies. But ofc this is super dumbed down.
This should be pretty easy to test in psykhanium tho. Literally just walk up to puff each elite and see what pops up. 
But I do feel like overall flak is way better considering it really helps with horde and that +25% dmg makes a big difference wherever it works. While the +10% elite would help with oggies, it’s not like they’re going to go down much faster either way from something like purge.
Edit: Btw I’ll go and test this at some point soon I think. That sheet is all about helping me decide on my perks etc. at a glance, that I didn’t have those base types for flamers & DoT’s there is a huge oversight I really need to fix.
Edit 2: I tested the base types & updated the original image in my first post. Flak covers 43% of horde, 1 special, 60% of elites, and possibly the bosses (couldn’t test that). Based on this, at least on paper it would be better even just for elites alone. Unless you’re specifically going for +10% dmg vs. Dreg Ragers, Bulwarks and Reapers at the cost of -12% dmg against every other elite.
But yeah, flak is a popular dmg type for a reason. c: