The difference in sound in my system with Beolab 1s as rears compared to 4000s is amazing.
There's a HUGE difference between the 1's and the 4000s. Less of a step between 9s and 5s though, unless you really intend to push the 5s hard. The primary difference is that the 5s simply take up far more space in the room - they'd dwarf my BV10-46, for example.
Basically the 9s were a replacement for the 1s, so using 1s is like 9s as rears (although without the alts).