multi-site/network stores performance dilemma


I am about to start working on a multi-site network of stores and have purchased a premium membership. However, the developers I am talking to are saying that if I ever reach a point where a store wants to put 1000+ products, categories etc. on my network (a great problem to have) – marketpress will never scale like opencart or magento.. specially since I would have to do all the backend work of finding product descriptions, categories, pictures etc for all UPCs..

Are there any examples of large network of stores being run with 1000s of products and categories using marketpress? I would like to validate this before I start coding.

Thanks for any inputs.