The CPU of the RPi 2 is good enough, some Synology NAS systems run fine with lesser cpu and memory
Whether it will be any good will depend on the I/O channels. SATA, Ethernet can be added . But since in a Raspberry all has to go the USB bottleneck it may be a bit slow and not comparable with a Synology NAS at all. I can see disk access fighting with network access on the USB port ...
A simple Synology NAS will cost just over 100 euro, and includes a case, power, room in a cage for one or two SATA disks, USB 2 ports, fast ethernet, cooling and a supported DSM.
A RPi2 with USB SATA, case, power, disk cage will cost less?
So for the price-performance and time to invest, not a good idea. For the fun of it, of course! And if DSM will not run, then there are other NAS solutions for the RPi (with the same argument: slow).
http://duinorasp.hansotten.com for Raspberry Pi and Arduino and ESP8266
http://retro.hansotten.nl for retrocomputing with the 6502 cpu