That setup sounds marginal to me but there is a good chance you’ll be OK. Also a small chance you’ll be wishing for something warmer.
I have no experience with quilts so keep that in mind …. but I have spent 80+ nights between 9500’ and 11500’ in the Unitas over the past decade, most of them in August.
With an inflatable pad rated at r4.5 and a 20* down bag I am comfortable most nights but often am wearing silk base layers and a beanie, sometimes socks.
Less than ten of those nights had temps stay 40* or above with probably 50 or more nights getting to freezing or below. The coldest I recall was 22* at 10500’.
There can be some high winds and heavy t-storms there in August even into the night. And have been in one snow on August 23 where there was 4” at 9500’ when the sun came up, not sure how much more accumulation was up high.
The weather is variable just like any high country, and the days can be quite warm in August even up high, but the night/early morning temps seem to be mid-30s pretty regular.
Sorry for the long reply. Just wanted to offer some experience with actual conditions so you could judge your gear relative to how you sleep.