Hi Trevor,
You should be OK at that resolution, (I did
this at 360 mm FL) but the short 5s exposure was the key (I used 5 & 180s *I acquired 20s which looked like a clump and did not end up using this data but only because I had added spikes to it*) and then HDR composition...which was such a pain to do and was massive trial and error, that I swore never to revisit it, and looking at the weather I probably never will...but if you cant ''window'' it to see it at 10s then go to 5s.
Cheers
Gary