Alborg,

I view all of this "be careful what you wish for" as a red herring. The AC database for each customer is only GROWING not shrinking. As users request more features more tables are going to be added further INCREASING the amount of data the system will have to store.

Your suggestion seems to be to run it as is and HOPE you never reach that threshold. Hope is not a strategy when it comes to software design and architecture.

I understand the misgivings of those who have dealt with poor implementations in switching from Access to SQL Server: that was not because SQLServer is bad, but because the developers were not sufficiently versed in the new product and did not have the requisite skillset to make such a change. Please read the last sentence again!
I posted the following a few months back. I am posting it again for you.

Here is an excellent Article that describes issues with the Jet Engine:
http://www.aldex.co.uk/sqlserver.html

In Microsoft's own words....
The following comes from Microsoft article Q300216.

"Microsoft Jet is a file-sharing database system. A file-sharing database is one in which all the processing of the file takes place at the client. When a file-sharing database, such as Microsoft Jet, is used in a multiuser environment, multiple client processes are using file read, write, and locking operations on the same shared file across a network. If, for any reason, a process cannot be completed, the file can be left in an incomplete or a corrupted state. Two examples of when a process may not be completed is when a client is terminated unexpectedly or when a network connection to a server is dropped.

Microsoft Jet is not intended to be used with high-stress, high-concurrency, 24x7 server applications, such as Web, commerce, transactional, and messaging servers. For these type of applications, the best solution is to switch to a true client/server-based database system such as Microsoft Data Engine (MSDE)[SQL Server Express] or Microsoft SQL Server. When you use Microsoft Jet in high-stress applications such as Microsoft Internet Information Server (IIS), customers have reported database corruption, stability issues such as IIS crashing or locking up, and also a sudden and persistent failure of the driver to connect to a valid database that requires re-starting the IIS service."

Performance
With Access all tables involved in a form, report or a query are copied across the network from the server to the client's machine. The tables are then processed and filtered to generate the required recordset. For example if looking up details for one particular order from an orders table containing, say, 50,000 records then the whole table (all 50,000 records) is dragged over the network and then 49,999 of these records are thrown away (this is an over-simplification since indexing can be used to mitigate this to some extent). Contrast this with SQL Server where the filtering takes place on the server (if designed properly) and only 1 record is transmitted over the network.

This can affect performance in two ways. Firstly SQL Server is highly optimised and can usually perform the required filtering much more quickly than the client machine and secondly the amount of data sent across the network link is vastly reduced. For most databases the main performance bottleneck is data transmission over the network hence reducing this can give a really dramatic improvement in performance.

Predicting likely performance improvements is very difficult .

"The illiterate of the 21st century will not be those who cannot read and write, but those who cannot learn, unlearn, and relearn." ~ Alvin Toffler