I've moved a couple to Premium and there are a few minor gotchas (need to re-do all SOTAMAS90 ODBC connections to point to SQL on every workstation if you have anyone using an ODBC connection for a report NOT on the Sage menu).
However, I think for someone with a large number of transactions that Premium is the way to go. My customer who had similar issues with speed called me to say that they indeed noticed a slight performance boost.
I always caution the users that they should not expect a night/day difference in speed for transaction processing. Where they will see the pickup is anything connected via ODBC. As Sage seems to put less and less attention into releases I predict we will create more reports to meet customer needs.
I automatically suggest Premium if (a) the customer doesn't have modules like WO which aren't read yet and (b) the customer has large table sizes, years of transactions, multiple companies, etc.