Reading the following link will answer a lot of questions for this thread I think.
I need to read it again in more detail, but in effect it is saying as long as your nav system is meeting the nav accuracy required for that phase of flight (SID/STAR, MTAP) at the time, then your good to go ! Irrespective of you nav suite. Please lets read it in more detail and correct me if I missed the boat.
P.S.
It also has the reference also about runway updating.
http://www2.faa.gov/airports_airtraf...1/aim0102.html
Or
GOOGLE 'RNP certification' and its the 3red one down.
P.P.S.
The A320 FCOM limitations section states ;
FMGS lateral and vertical navigation has been certified for after take-off, en route and terminal area operations, for instrument approach procedures (except ILS, LOC, LOC-BC, LDA, SDF, and MLS), and for missed approach procedures.
Then it goes on to say what nav accuracies it has been demonstrated to with GPS primary, with or without warious combinations of; AP,FD and NAV MODE.
Then the most interesting bit;
Without GPS PRIMARY the accuracy has been demonstrated, provided the appropriate RNP value is checked, OR entered on the MCDU, and HIGH ACCURACY is displayed.
For the non-bus drivers 'HIGH ACCURACY' is just a function of the MCDU that 'checks for you' the difference between the required and actual nav accuracy. Then tells you if you have HIGH or LOW ACCURACY. i.e. so you don't have to check it your self constantly during the STAR. Its all automatic for normal operations using the database anyway.