Observe the cpu usage of nice +5 job: it's ~1/10 of the nice +0 job. End one of the tasks and the remaining jumps back to 100%.
Nice'ing doesn't limit the max allowed cpu bandwidth of a task; it only matters when there is contention for that bandwidth, like running two tasks on the same CPU thread. To me, this sounds exactly what you want: run at full tilt when there is no contention.