First of all, even if Alfresco is using SolR, you don't have the access to all the functionnalities. I don't think that creating multiple collections is possible in the Alfresco product.
The closest thing to what you are talking about is sharding which enables you to split your index in smaller ones to improve performance : http://docs.alfresco.com/5.1/concepts/solr-shard-config.html.
The documentation explains well the mecanism, you have the choice between manual or dynamic sharding.
Finally, if you are looking for better performances, alfresco replication is another lever to improve you alfresco responsness.