The rMBP is coming out whenever Mavericks gets released; most likely in the fall.
The reason is as follows:
1.)
The next iteration of 15 " rMBP will have modest upgrades. Maybe 5-15% increase in CPU performance and up to 20% increase in GPU performance. In OS X 10.8, it will only in a very minor sense improve the lag. There might be some energy savings but this is touted as a "performance laptop", so if there are any at most it will up to 9 hours of battery life instead of 7.
The only performance boost will be the upgrade to PCI-e SSD, that will be 2x faster than this generation and Iris Pro to the 13; which will bring it to the current 15 level in terms of GPU.
1a.) Releasing in the fall gives Apple a chance to release old inventory and make extra profit on the reduced processor prices; while building up new Haswell inventory. They can also skip the whole back to school special on the Haswell rMBP => more profits.
2.)
With OS X Mavericks; it will claim to fix most of the lag issues, while increasing battery life. The gains mostly will be in the software and old ivy bridge rmbp will get the same benefit. Also less memory will be used.
In short, there will not be a big change in the upcoming rMBP. It marketed as a "pro laptop" so Apple will keep the discrete in the 15 and upgrade the 13 to use Iris Pro (*13 will see the biggest performance bump due to the GPU).
side note: You can buy a used base 15 rmbp for $1500 or around $1700 refurbished. Not a bad deal. I use a non retina resolution to solve my performance issues with the 15 rmbp.
That is my prediction.
