Parallel differential signals
I would say that at 5 meters you stand a good chance of using a straight I2C hook-up and getting reliable performance IF:

A. You are very careful with your grounding between master and slaves. (E.g. Make sure there are no ground loops and that your shielding doesn't contact any non-system metal in their routing. (See E below)

B. You minimize the pull-up resistor on SDA - 1K or less.

C. You minimize the clock frequency to as low as you can make it without impeding the data flow that you actually need between the master and the five slaves. I2C is a synchronous protocol, so there is no low frequency limit for the clock. The limiting factor will likely be the I2C controller or firmware in the Arduino processors.

D. Buffer the SCLK line to each slave with something like a 74HCT125.

E. Use low capacitance cable. E.g. Unshielded Cat-5, using the extra wires as your ground connections.

You can prove this out on the bench before you deploy it into your final system. 

Performance will depend to a large degree on the electrical noise environment in which your system needs to operate.

It's a bit of a gamble, but it may save you the cost and complications of using the differential drivers and receivers. I have run I2C this way over approximately 5 feet of twisted pairs in a past project. One pair for SCLK, the other for SDA with the other wire in the twisted pair connected to DC Common. In that case the master and slaves shared the same power supply. The clock frequency was around 10 or 20 KHz.