XI 3.1 Web Inteligence MDX condition v SAP BW query filter

My colleague seems to think that if I restrict the BW query it will perform much quicker than if I filter using the MDX in a condition. I haven't proved this yet but would be interested if anyone can confirm that restricting in the MDX would/should perform as well as in the BW query.

Example:

I have 5 years of sales data. I then create a BW query that only returns data for one year. Build a BO Universe over that and hey presto....

If I then repeat this but do NOT restrict the BW query but add a query condition that restricts query to return the same year.