U think that upgrade to V9 can grow u're client connection ?, I think it will be less expensive in ko thant v6. In v9 u can store data in different area. These area can have different block size and then i think that it will be better for u.
So i think that an update from v6 to v9 need to be analyzed.
For me you have to use the new possibilities of the progress v9 databases and make a correct .st to store efficiently data and index.
U don't specify the type on client connection you use, so my answer is for client server connection. So you can also use terminal server connection or citrix metaframe to control u're connection.
Hope it's help