Is the ReadBuffer property equivalent to

set arraysize nn

in SQLPlus?

Yes. It controls the number of records fetched in one network roundtrip.

I asked this because reading the last Kyte's book
I saw that this setting impacts performance even
in lowering consistent gets on the server!
This quite surprised me.
I always thought about this in term of network related issue only (reducing round-trip, band-width consumption and so on)