Marotzke and Forster have published a response to Nic Lewis's critique of their paper. It can be seen here, at Ed Hawkins' Climate Lab book site. Here's the start.

No circularity It has been alleged that in Marotzke & Forster (2015) we applied circular logic. This allegation is incorrect. The important point is to recognise that, physically, radiative forcing is the root cause of changes in the climate system, and our approach takes that into account. Because radiative forcing over the historical period cannot be directly diagnosed from the model simulations, it had to be reconstructed from the available top-of-atmosphere radiative imbalance in Forster et al. (2013) by applying a correction term that involves the change in surface temperature. This correction removed, rather than introduced, from the top-of-atmosphere imbalance the very contribution that would cause circularity. We stand by the main conclusions of our paper: Differences between simulations and observations are dominated by internal variability for 15-year trends and by spread in radiative forcing for 62-year trends.

Unfortunately, when they continue to the section called "Specifics" I can't actually see any mathematics that purports to show that their original regression model was not circular. My impression is of handwaving. Steve McIntyre, in the comments at CA seems to have reached similar conclusions:

I’ve done a quick read of the post at Climate Lab Book. I don’t get how their article is supposed to rebut Nic’s article. They do not appear to contest Nic’s equation linking F and N – an equation that I did not notice in the original article. Their only defence seems to be that the N series needs to be “corrected” but they do not face up to the statistical consequences of having T series on both sides. Based on my re-reading of the two articles, Nic’s equation (6) seems to me to be the only logical exit and Nic’s comments on the implications of (6) the only conclusions that have a chance of meaning anything. (But this is based on cursory reading only.)

I guess we will have to wait and see what Nic Lewis makes of it before reaching firm conclusions.