We have a very good experience in both x-cart (5 years) & magento (2 years) shopping carts. You can find more at
www.mercuryminds.com/portfolio.php
Recently, we migrated a system (dot net based) to magento (60K customers, 38K products, 120K orders) with multi store option. Similarly, we have migrated a system (dot net based) to x-cart (80K customers, 22K products, 150K orders). These two sites will give an idea the capability to handle & how good we are in understanding the database structures well. Please note that while migration we took care of the statistics too.
We can definitely perform this migration (design changes), data migration if needed. Please email me your contact details to
sales@mercuryminds.com.
sent you a PM.