It's too hard to test otherwise, but I see a noticeable increase in acc from day/weather. If I were to eyeball it, I'd put it around +5-10% accuracy boost.
Since when is 5% noticeable?
The test should be fairly simple.
Pick a neutral day or the week.
Head to Sea and grab a Goldfish.
No gear/Grimore > spam paralyze. D skill should result in a lot of resists.
Add Hailstorm (no obi) > moar paralyze.
Hailstorm + Obi > still moar paralyze!
Because the goldfish will never attack you, it's one consistent mob, for as long as the test will take. Might have to chase it down every now and again, but you could do this all day, really.
Paralyze is 6mp with a 10 second recast = 36mp/min.
With Arts/Weather/Gear/Klimaform, SCH is pretty well buffed to provide a plethora of "baseline" tests (ie. D => B => A skill ranges).
I don't think Hpemde have elemental strengths/weaknesses, which could make them good test subjects for multiple enfeebles.
This can also be tested in reverse, throwing up Firestorm.
It'll be really hard to figure potency, as there is no "attack", so nothing can be stopped.
My SCH is only 44, which makes it hard for me to just "run and up do this".