I'm trying to set the usePowerOf2Sizes flag to a collection. I tried
from a mongos:

mongos> db.runCommand(

{collMod : "test" , usePowerOf2Sizes : true }

)
{
"raw" : {
"rs_test/db1-test:27017,db2-test:27017" :

{ "usePowerOf2Sizes_old" : false, "usePowerOf2Sizes_new" : true, "ok" : 0, "errmsg" : "unknown option to collMod: $queryOptions" }

},
"ok" : 0,
"errmsg" : "

{ rs_test/db1-test:27017,db2-test:27017: \"unknown option to collMod: $queryOptions\" }

"
}

On the replicaset master, setting the flag works:

rs_test:PRIMARY> db.runCommand(

{collMod : "deviceInfoView" , usePowerOf2Sizes : false }

)

{ "usePowerOf2Sizes_old" : true, "usePowerOf2Sizes_new" : false, "ok" : 1 }

JFYI: This cluster has been upgraded from 2.4.