I would get in touch with Chris at Dasboata props. props are what he does and he will have your best answers. there are other prop guys as well, Like Bob Austin and G-force. But Chris is who I know and have used. The 26 Zenoha will work fine in the Thundercat. All Of the gas motors have the torque to spin big props and push bigger boats. But RPM's is where your speed comes from. so the stock RPM's will still get you in the mid 40's. But a full mod Zenoha getting 18k plus RPm's will get you into the 50's. Over all I still feel the weak links in boats are the users. I have seen plenty of model boats with stock motors in the hands of the right guys go faster then lots of modelers with Modified motors and even known faster hulls.
Some of the stock motor racing classes get very good MPH speeds.