Ideally the Docs like to see an AHI value less than 5.
An average pressure of 18.2 is fairly high, from what I have gleaned from the board... I'm glad you're giving BiPAP a shot. My regular S9 Autoset only goes up to a pressure of 20, so if you have a similar machine (I believe the System One is similar...others can correct me if I am wrong) and are running close to the high end on pressure anyway, there's not much left to help for "bigger" apnea events. The BiPAPs have a wider pressure range and should be able to help out a bit more.
If you take to the BiPAP as well as you have the CPAP, you'll probably drop the AHI value a bit more and feel even better after a really good night's sleep.