SAS is an interface, not a type of drive. You can get SSD drives with SAS interface easily.
You also write "SAS which are 15k5 RPM", but SAS does not dictate rotational speed (nor, as mentioned, that the drives use spinning platters at all). They are usually higher RPM if they are regular hd's as it doesn't make much sense to buy SAS controllers and SAS drives if you're going to opt for cheap consumer level performance. But my usual source of drives have both 7200 rpm and 10k RPM SAS drives.
Spending on SSD's for SAN/NAS storage would make absolutely zero sense unless your connectivity to said SAN/NAS is extremely low latency and high throughput - it'd be like burning money, so I very much doubt that's the bottleneck.
You also write "SAS which are 15k5 RPM", but SAS does not dictate rotational speed (nor, as mentioned, that the drives use spinning platters at all). They are usually higher RPM if they are regular hd's as it doesn't make much sense to buy SAS controllers and SAS drives if you're going to opt for cheap consumer level performance. But my usual source of drives have both 7200 rpm and 10k RPM SAS drives.
Spending on SSD's for SAN/NAS storage would make absolutely zero sense unless your connectivity to said SAN/NAS is extremely low latency and high throughput - it'd be like burning money, so I very much doubt that's the bottleneck.