I cant speak to the storage or specifically the 8a as I went with the 8.
GrapheneOS is snappy though. Never had a hang or experienced any lag outside of a segfault one time that crashed the phone. There was an update pushed out the next day and haven't had any issues since.
GOS is under constant development and fixes for things seem to be fixed pretty damn quick.
GOS also tells you when an application has memory issues like buffer overflow which could be a security risk.
GOS puts you in control of your device in so many ways, sandboxing apps and user profiles, installing google play services to a user or not, restricting app permissions.
It is hands down the best phone OS i have ever used and i have done a fair bit of custom rom installs on my galaxyS4 back in the day. The best part is you dont even have to root it.