Normally a R card Max Level have 440 Charm (Power), a SR card only need up to Lv30 to have same Charm (so it doesn't really need to break limit). The problem now is Skill Level, because Unit Skill is the most important, so it's a big disadvantage if you can't rise it up to Lv15.
Though, the wikia not say how much bonus with Unit Skill higher than 10, but from Lv9 to Lv10 it get 30% bonus, so maybe it's 30% for each level, then Lv14 (highest possible you can get right now) "maybe"...is 470%.
With a R card Max Level is 55, Team Unit Skill Lv14:
440 x 470% = 2068
And a SR card Max Level (Lv50 without break limit), Team Unit Skill Lv10:
600 x 350% = 2100
Oh my god, did you see that? Event with lower level of both card and Unit Skill, SR card still stronger.
Now what if you have a SSR card?
SSR card Max Level (Lv60 without break limit), Team Unit Skill Lv10:
900 x 350% = 3150 (Oh my God...My God...God...
)
I'm sooooo envy with people that got Sayumi SSR card. If right now I got a SR or SSR card, I still can get Team Unit Skill Lv14 (8 cards Lv15, and one card Lv10, total is 130 -> Unit Skill Lv14).