The Air has a very small heatsink/fan setup as well. The 15" rMBP currently has the cooling capacity for a 45W CPU and a 45W GPU running at the same time. Dropping that down to 47W total thermal load will be a big reduction in maximum heat output. Unless they completely gimp the cooling system at the same time it should run much cooler.
47W is not the worst case yet. TurboBoost may still increase that by a lot. And I'm not sure if Iris Pro depends on TurboBoost to get its higher performance, but I know the HD 4000 does.
And the 15" rMBP definitely doesn't have to deal with max CPU and max GPU load at the same time. If both are putting out too much heat, then the CPU actually throttles down to lower performance. That means no TurboBoost, and possibly even lower performance than nominal (so say, if the CPU is 2.3GHz, then it runs at 1.6GHz).
You also have to take ambient temperature into account.