sounds to me like the basic problem is the fact the sql server i/o is shared with i/o from other types of servers. the i/o patterns for SQL are radically different to say a file server and the two should not go through the same i/o channels.

you would get the biggest bang for your buck by seperating them out. that would probably give you improvements in SQL performance as well. I don't think adding more luns if the i/o is still mixed is the best option.