(1) how important, performance-wise, is it to separate out these files?
->it is easier for management in case you decide to reorganize storage. Logfiles have mostly sequential reads, datafiles are mostly random io. Logfiles could be moved later to fast sequential disks (smaller capacity but more) and some datafiles to flash-storage (great reads)
(2) is there a benefit in having the data and tlog files on separate windows volumes (ie, storage-based LUNs), based on the fact that all the data is spread across the whole of the storage array anyway?
->Same as above, allows for easier storage tiering/isolation/monitoring/redirection when seperate
(3) will having separate volumes (ie, storage-based LUNs) mean that there will be separate paths for the data to travel to and from the storage array through the SAN fabric, will this help with performance, and if yes, how?
->It probably generates seperate paths, depends on the storage/fabric if there is some quality of service. There can be paths with high priority (like fast fabric) and with lower priority (slower fabric in case fast gets overloaded).