I'm not an expert here, but weren't people complaining about sluggishness in certain areas on the current gen rMBP? And Apple's answer to that is to take the dedicated graphics card out and replace it with the Iris? Logic dictates that they will attempt to fix the issue, not make it worse. A dedicated chip in the rMBP has to happen.
But like I said, no expert here.
Also, I've been kind of waiting for the rMBP. I don't need the retina display, but at the same time the cMBP is not usable for me at stock config and I would need to upgrade it anyway. So essentially after upgrades and stuff both cMBP and rMBP come out to be the same price... and I'm totally confused on what to buy. I may just get buy a desktop and an MBA if the graphics performance doesn't increase in the rMBP and the cMBP doesn't get a refresh.