Because it is still same engine, there will be no performance difference. Just the resource limits can have impact to the performance in this case and it means just if you need more than 4 CPUs you will have performance difference (standard use max 4 CPUs). Other features which are in Enterprise version are not used in NAV.
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Customers who decide to use enterprise version should be on 64 bit anyway so resources etc. is not a limit anymore.
http://www.microsoft.com/sql/prodinfo/f ... tures.mspx