First of all the 300mm is absolutely amazing, and you can read about it, and some other MF telephotos on my website. I've been in Costa Rica shooting the Mamiya 300/500 APO lenses as well as the 300mm f/4.5 AF and the 300 AF's work great. Last time when I was here with the Hasselblad 300mm I got one great picture and I was hooked to Medium Format telephoto....
Also I absolutely love the focal length of 210, just saying
But on to your question, I believe there has yet to be the "firmware" upgrade needed to allow AF with the Hasselblad lenses and the the 1.7 TC....so this is something big to consider especially if you are using AF of course....this information may be a bit old but I BELIEVE it is still correct, someone please correct me if I am wrong.
Using the Mamiya 2x TCN with the 300/500 APO's, I am amazed by the image quality you can get, and the main cost is in some vingetting and of course the slower aperture. In most situations where everything else goes well, the image quality seems to be very similar between the TC and the non-TC...and if an older Mamiya TC can rock on a 80mp DB, then I suspect that the Hasselblad TC should be excellent.
These are just my thoughts on these focal lengths, and MF TC's, I am sorry I have not had any direct experience with the 1.7x TC from Hasselblad with their lenses.
It would certainly be an attractive option if the 300mm Hasselblad could AF with the 1.7x TC on a Leica S2 (because of its crop factor) but if the firmware upgrade has yet to come to H bodies, then I can only imagine it will be substantially longer before it comes to the S2/H-Adapter.