I worked for a company that builds IMS/DLI to DB2 conversion tools and we always got in the middle of this religous debate.
IMS is a hierarchical database structure which means that parent-child relationships are faster for sequential traversing. e.g: Show me all children for parents whose ages are between 25 and 40. A relational database system cannot automatically satisfy such a request by the nature of the table structure. It must be done via program code of SQL joins and other such queries.
However, given the same scenarion if you wanted to find all children whos ages are between 5 and 15, DB2 only needs to maintain position in the child table and does not have to traverse the tree back up to find the parent key.
Design, Configuration and Tuning are always needed in any database environment and can radically affect performance also.
Your consultant may be right in certain cases, but it is (as always) up to you and your business to decide which engine is the better. If you want open, future-proof technology go with DB2. If your business's data model is hierarchical and performance is paramount then IMS may be the better path. You will not however have as plentiful a resource pool of IMS personnel to draw from for staffing and that is also a concern.