I'll answer your questions, however, operating Riak in this manner is strongly discouraged as Riak assumes identical capabilities among nodes.
You could possibly have wildly varying performance characteristics for operations against your node. In general, the "weakest node" in the system could affect operations throughout the cluster. For instance, during the PUT phase of an operation, a replica of the data could be routed to the weakest node and the duration of that operation could affect the entire PUT operation based on the PUT operation's quorum value.
No, the cluster assumes identical hardware.
There really is no way to compensate for this.