Xyrem has a very short half life - i.e. it is metabolized very quickly. When it is prescribed for Narcolepsy, it is usually prescribed to be taken once right before bed, and then a second dose about 4 hours later. If you are not taking it twice a night, I think that you should talk with your doctor about it. Splitting the dose will almost assuredly get you more sleep with less side effects.
Your increased AHI may be due to the fact that Xyrem (GHB) is a CNS depressant and can reduce respiration, but it sounds like your CPAP is working to keep that under control.