I expect that Outlook treats those files the same as files which just reached the size limit. As far as I know the registry key tells Outlook to behave in a certain way as soons as the size is larger than the treshold.
This is pretty easy to test in on a computer by setting the registry key to a low value.
Yeah good call I will get testing. So from what you are saying There shouldn't be any problems in opening the larger PST's as outlook will simply treat them as pst's that have reached there limit and then start to creat antoher for new additions?