The following commands show that volume top succeeds even when a brick's ancestor directory is given instead of the brick path.
When an ancestor directory is given, it will display output of the first brick found in the recursive directory search.
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root@raj-pc:~/glusterfs.git# gluster volume info
 
Volume Name: v1
Type: Distribute
Status: Started
Number of Bricks: 3
Transport-type: tcp
Bricks:
Brick1: raj-pc:/home/raj/bricks/a
Brick2: raj-pc:/home/raj/bricks/b
Brick3: raj-pc:/home/raj/bricks/c

root@raj-pc:~/glusterfs.git# gluster volume top v1 read-perf bs 1024000 count 100 brick raj-pc:/home/raj/bricks/a
Brick: raj-pc:/home/raj/bricks/a
Throughput 3470.60 MBps time 0.0295 secs

root@raj-pc:~/glusterfs.git# gluster volume top v1 read-perf bs 1024000 count 100 brick raj-pc:/home/raj
Brick: raj-pc:/home/raj/bricks/a
Throughput 3487.98 MBps time 0.0294 secs