Very nice panning work! I tend to avoid slow shutter work myself but only because my lens is a tad heavy. Keeping the images sharp is just a bit too hit or miss.
One piece of advice I would give with slow shutters though is to shoot for your subject. Motion blur is all well and good but it can look unnatural if over done. I think you have hit the nail on the head with this one and got the shutter speed perfect. But so many times viewing forums I see images of an aircraft such as the Magister or similarly powered types and they have gone for extreme slow shutters. It just looks wrong to see an aircraft like this looking like it is going supersonic. If it were a jet however it would look fine and you can pretty much shoot as slow as you can handle. I feel the same with shooting warbirds too, I would never really want to go below 1/100. I don't even like full radial prop blur in images. For me it just looks a bit weird and you inevitably trade some sharpness for it. I guess it's just a personal thing.
Anyway I do think you have got it right here though it looks just about right for my liking.
Thanks Dan! luckily at Shuttleworth I can use the 18-300 which is quite light and more suited to this sort of thing, but your right its so hard to get a good sharp image, I have a number where the front of the aircraft is sharp but the tail is very off. Weirdly the miss rate was only slightly worse than when I switched down to 1/250 for both the Barnstormer's tail chase and Hawker biplanes, but then I was surprised with how poorly I did on these 2.
Defiantly agree with you here, Its very easy to get caught in the "how low can I go" game and forget your subject. I quite like the Disc prop but only in certain shots, but i do get what you say as the do look a bit odd, funnily one of the reasons night shoots have never relay interested me. At the end of the day its as you say a personal thing.