Sadly, the Balance One was actually originally rated for 1000Mbps, but we found under certain conditions (Apple Mac devices, if I remember correctly) we had a rather steep falloff in throughput that we couldn’t compensate for.
( see Cannot Achieve Gigabit Throughput on WAN / Balance One - #27 by soylentgreen )
As a result, we downgraded the throughput specs for the Balance One series, as we couldn’t guarantee it under all reasonable deployments.
Next step up would be, as you’ve noticed, the 305, which should handle your expected service levels.