I agree that Itanium is right, but with regards to this:
I would not get rid of the old pages, but you can change the metatag to noindex after the new pages are indexed.
If the old page is redirecting, the noindex metatag will not be seen. I would not change anything, just leave 301 in place forever.
Here is the timeline of what is happening in cases like this:
1) Google sees new URLs (from the reedesigned site). Google becames very busy crawling all these new URLs. This can later on be seen as a spike in crawling in WMT
2) Google indexes new URLs
3) Existing URLs are crawled at a standard rate, or sometimes even the crawl rate of existing URLs slowed down. So Google now has both, new URLs and the old URLs in its index. This is the stage where you get multiple SERPs entries and duplicate titles/meta descriptions reported in WMT
4) As Google continues to crawl OLD URLs, it sees 301 redirect to a new one. From what I have seen, sometimes it processes redirect straight away (old URL drops from SERPs and duplicate title/meta desc disappears from WMT), but I have also seen that in a number of cases Google does not process 301 redirect immediately - sometimes for a while it still leaves the OLD URL in SERPs despite having it crawled and seeing 301.
5) Sitemap stays with a very few pages reported as being indexed for a little while, even after almost all new URLs have been indexed. If you wait long enough, it will eventually be reported correctly.
In my experience, you need to wait from a few weeks to a few months, depending on the size of the site, how often the site was crawled, how often its pages used to change, etc. Sometimes most of the important pages will revert to new URLs, but there will be some old pages that still keep showing under old URLs for quite some time.
Regarding 10%-15% link juice waste, we have to go on what Google said as any particular number is almost impossible to prove. When the site is redesigned and URL change, then it is often that the other changes done and other factors have much greater impact on what will happen in SERPs afterwards than this 10% - 15%
Better IA, getting rid of other errors, adding some new content, removing some thin pages are usually all part of site redesign which would impact how the site ranks afterwards.