Of course, the primary loss would be from links pointing to the page from external sites. If you update your internal links, there would be no loss since Google would eventually discover all of the changes. I think we have to assume that the 15% figure was somebody's estimate, anyway. You're bound to see a drop in performance from that page until Google absorbs it all, but I don't think it's worth worrying about. If the move makes sense, then PageRank should not be a factor in your decision.
It's really most worth worrying about when you have a strong, authoritative link pointing to the old version of an internal URL. I have found it is worth investing the resources in getting those linking sites to change the URL - and also getting a few new backlinks to your best internal pages soon after the change.
But the majority of the loss really ends up not amounting to much - especially today when the algorithm goes so far beyond PR.