How accurate is Speccy in terms of temp readings?

The temps are read at three places usually. One is on motherboard below the cpu socket. there's temp diodes for the cores and temp diodes for IHS (integrated heat spreader, the HSF sits on this).

cpu package temps seem higher than core temps and this is because the package temp is read at the mobo socket and is warmer. the temp diode at the IHS-HSF contact will read the least temp as that dissipates the heat fastest.

the most important temp is the core temp. the further you are from reaching it's threshold the cooler you are running.