I've not shot Retropan at all, but my starting assumption is that all films are really 1/2 box ASA with normal agitation development. In this case, it's 1/3 slower than that, which is well within meter and temperature tolerances, so it's not a shocking number. The only way I've ever found to get film to hit full box ASA and deliver decent Zone III shadow detail is if I do extended development like N+++++ or semi-stand/EMA low agitation development.
I have shot tons of Fomapan 200, and a bit of the 100. They love to build contrast really fast, and in some unpleasant ways. For low contrast scenes, this works to your advantage and you can get wonderful results with semi-stand or EMA. But for scenes with 7 stops or greater SBR, the long development starts to blow out highlights, or least makes them dense and hard to print through. I am currently exploring divided Pyrocat-HD for those situations. It's great for very long SBRs, but I am still dialing in what to do with a 7-8 stop range of light in this regard.