Yea EGIP was two west facing sidings and no loop around the back of Meadowbank
The new outline plan is to add a third line as far as Powderhall Junction. This is presumably reusing the trackbed via Meadowbank, but it's never explicitly stated.
Benefits are being able to dispatch a North Berwick or Borders Line train onto the 'loop' and then have a fast overtake it before Craigentinny, and being able to dispatch an empty East Coast set going to the depot before the depot is ready for it.
Currently 'platform blocking' is a bit of a thing at Waverley as you can't send an EC set to the depot early to free up a platform. It will block the line at the velodrome until the depot is ready to take it in and then both lines for 5-8 minutes while crossing into the depot.
It's really subtle operational stuff
Details (or lack of) are at https://www.networkrail.co.uk/wp-content/uploads/2016/11/Scotland-Route-Study.pdf (via here) - page 61 along with every other major project you're likely to see thrown around in the next 20 years.
There's a bit of commentary as to further justification and timeline in the appendices starting from page 81ish