I believe what the manual is saying is that when the filter is empty, the distance should not be more than 10cm. Otherwise it is very difficult to prime the cannister filter.
However once water has filled the cannister filter, it should be no problem running it at a distance of more than 10cm.
I have tried it many times especially during water change. Many times I had to attend to other things when my 2" tank is only 20% full. If it takes me more than 1 hour, I usually turn on my 2080 even though it is only 20% full so that the nitrifying bacteria will not die.
2080 is a great filter. However it also has its draw back. When it is fully filled with water, even with wheels it is very heavy and difficult to move. Unlike the FX5, you could drain the cannister filter first before moving it. One solution to this is to put the 2080 on a plant mover which is available at most nursery or Bunnings (< A$20). Be very careful when moving a fully filled canister filter which could weigh about 30kg!
Another potential draw back is when cleaning the outlet of 2080. The filter has an inbuilt indicator supported by a very fragile spring. If not careful enough like me, once the spring is stuffed, the flow rate indicator will not work properly.
