What elevation? I was in SW Colorado at 10,000 ft during the 2nd week of October and woke up to 6° one morning. With a 20 degree Marmot bag, sleeping pad and a 4 season tent I was okay but I would have frozen my butt off with just a tarp. Not sure about the end of September but probably better safe than sorry.
I like a UL bivy to keep the dew/condensation off my bag and to protect my inflatable pad from the ground. Take a look at the TiGoat Raven Omni bivy. It's 8 ounces of cheap insurance and should help keep you a tad warmer if temps drop into the low 20s.
The last couple of years, i have noticed lows in the 20s. Highs can be in the 60s. It hasn't snowed much. But 5 years ago we had 18" of snow after a very nice day. The weather can change drastically. Usually it's pretty warm in September and early October though.
A lot will depend on how cold or warm you sleep. We usually go 2nd and third week of September. Base camp is about 10,500 and spike camps are never below 11,000. I take a 30 degree bag and have not had any issues in Central Co.
Definitely take the bivy sack. Last year on our backpack elk hunt we camped at just under 10,000ft in SW CO ten miles south of the town of Del Norte. I used a tarp tent along with a good down sleeping bag. I was glad that I brought my bivy along.