The overall speed will be limited to a slowest link in the chain. If you have a 1GbE NAS, then SSD cache will not help to speed up data transfer because you are probably already reaching near maximum Gigabit speed around 100MB/s. If you have 10GbE connection already then adding the slowest thing in your setup is probably HDD speed inside the NAS, or HDD/SSD speed on your computer where data is transferred from/to. But SSD cache would certainly improve the 10GbE speed in most of the cases.
I hope this helps.