I would have thought 5 amp would be more than suitable, Ian.
When I went about wiring my layout I didn't bond the rail ends, I just used the fishplates to make a mechanical connection. I fed a dropper from the middle of each metre length to a ring bus around the layout. My thinking on this was, with end to end bonding you are relying on the soldered joints between each rail length for total continuity, thus when you get 40 metres away from your power feed you have 38 bonds and 76 soldered joints on each rail (+ & -). A break or dry joint in any of those soldered connections means either loss of power or a high resistance to the rest of the track from that point.
It was a pain in the butt wiring all the droppers, but that was done before I set foot outside, all I had to do once out there was solder the droppers to the bus, and for that I used a soldering gun more than capable of bringing the joint up to temperature. (it was 150watt and made light work of it)
I don't regret going down that route as I didn't fancy trying to bond the ends together at ground level outside (bad back, bad eyesight etc)
Sorry for going on so long, but if you take a look at pages 10 & 13 of Cherry Parkway you will see me doing the job.
George