My AHI has been averaging 2.6 for the past week, which is down from 3.6 the previous weeks. It's as low as I've yet managed to get it. As a general thing, I feel fine during the day. When I don't, however, it's more likely to be because I didn't get at least 7.5 hours of "sleep" (i.e., total time from lights out to waking up). That is, I "feel" more sleep deprived the next day when, for whatever reason, I get a short night's sleep than I do when the AHI is higher. I've had days when I felt great after a night of AHI=6.
It's also important to keep in mind that there are other things going on besides respiratory issues that can affect the quality of your sleep. My sleep study revealed a significant number of "spontaneous arousals", which are wake-ups or near wake-ups not obviously triggered by apneas, hypopneas, or limb movements. They occur for some other reason. Some could have been artifacts of the sleep study environment itself, but then again, maybe not. It's impossible for me to track and study these, but I have to remember they may be affecting the quality of my sleep.
So, I view a reasonably low AHI as a necessary but not sufficient condition for a good night's sleep.
My AHI seems to range from .5 to 4.5. For the last couple of weeks almost every night is at least 2.5. I don't necessarily feel any more tired after a 4.5 night than I do after .5 night. What matters more is how many hours of sleep I'm getting. Obviously I'm more tired after a night of only 5-6 hours than I am after 7-8. It's my own fault; I stay up too late sometimes on weeknights. ;-)
AHI numbers are relative to treatment. Compared to untreated numbers the AHIs of 5-10 that I got for the first couple of months were good. I was envious of the people reporting numbers below 1. Now that I have a full year of treatment under my belt, my ave. and 90% pressure numbers are decreasing and AHI is consistently below 1. So keep chasing those numbers, they will come.