May I ask where you see version 2.8? As I have seen some Chinese sellers saying they are selling a 2.8 version but I think they are just getting confused with APM 2.8 boards. The pixhack is based of the pixhawk V2.4.7 schematics which is the current latest version of pixhawk.
However in terms of the actual software features there is absolutely no difference between various versions of pixhawk and pixhack as the new versions are minor hardware changes to improve performance, or increase reliability, so all versions will have the same features such as terrain following etc… APM boards use a slower processor which is why they cant support all the features of pixhawk.
On a side we will soon sell a raspberry pi powered autopilot in mid Feb (which runs the same arducopter/arduplane) code, but these are still more tailored towards developers as they are abit more complex to use and larger than pixhack. I know 3DR are working on a new autopilot currently but not sure when it will be released/announced, but my guess is sometime towards the middle to end of the year only.