Hi Timbo,
I know Dave Amyot pretty well. He is (was?) normally working for ZimAfricas and is a very good PH. He guided my son (13) for a successful buffalo hunt and I have hunted hippo with him last year. So, no worries on that side.
Concerning the area in which you will hunt, as one post mentioned, there are some borderline operations in the area, operating out of quota. Some are genuine also, but there is a tendency to have SA operations coming in and out, "renting" Zim PH, shooting with clients and leaving asap with trophies. So a proper enquiry is in order and I would NOT hunt with a SA operation there (it is likely, very likely to be hunting on stolen properties).
Dave is a very honest person, so this would vouch for the legitimacy of the operation.
Just one word of caution, the Hwange/Gwaii area has buffalos, and some very nice ones, and lots of other game (incredible elephant area). But also two main herds (100s of buffalos) which come and go. I have hunted there and you can one day see hundreds of buffs, then nothing for a week or more. Depending on the season (read water in the Park or not), you will have a hard time being successful. I would personally not go there early in the season, certainly not before July, which is when lone bulls will start moving out of park. Also, be aware that this region had LOTS of rain this year and grass is going to be very high until late.
Hope this helps