I'm planning a 6'x2'x2' in a few years time since I've managed to get a lawn on my first. I'll be using nothing more than 3 or maybe 4 Eheim 2217s. I have a 2260 I can make up, that about 2000 litres and hour and 18 litre tub, but I'll probably not be using it. The 2260 and the 2080 are extremely awkward to service because they are so heavy.
The 2217 is rated at 1000 litres an hour, at zero head and of course no resistance with hose attached. I'm sure in normal use you probably be getting 800 litres an hour. In my experience resistance build up on canister filters also has a lot to do with how you layer the filter media because I've experimented on this in the past. In general the larger the cross section the water has to go through, the more difficult it is to choke a filter, because water following a path of least resistance have more options. Having fine filter foam at the bottom also does not help, nor does using a thick layer of it on top when you should be using just enough.
I'm not saying that the 2080 is overkill and baring in mind Clive and George have much more experience than me when it comes to flow in planted tanks, but for me I'll be just using 2217s.