From my personal experience: I couldn’t say that current method is really good. I don’t understand, why all members have to take a part in Wireframes and UX flows step. They are not professional as UI/IxD/UX designers, so why they have to spend time for that. Where is Visual Design phase? If you are going to make real user testing, it means that you have to prepare something real for that. Users don’t understand wireframes.
Google Venture suggests that you should have in your team CEO, Product Manager, Engineer and Marketer. Well … I would like to notice that it’s really expensive to spend 5 days for that. Of course, it’s super important to have this team for the first and second phases, but the rest of the things has to be done by professionals.