Have you ABX tested 320kbps mp3 against flac? If so, what track was it, and what audio equipment? If not, the difference you heard was the placebo effect of knowing it was flac.
All lossy codecs have some problem samples and cranking up the bitrate isn't a guarantee of transparency. Newer codecs tend to do better than MP3, and at lower bitrates. The only point in lossy encoding is to save space, so it makes sense to do it as efficiently as possible, and considering that, 320 CBR is actually the worst of both worlds because it always wastes space, and is often overkill anyway.
An AAC file from a good implementation of the encoder (e.g. Apples, or either from Fraunhofer including the Open Source FDK-AAC from Android) will usually (for most people, with most music) achieve transparency in the mid to high hundreds of kbps.
The material listened to always makes a difference, therefore unqualified arguments about "can always tell" or "never can tell" are always flawed.
