I consider the standard Corsa speakers to be poor quality. Actually the whole Audio system is poor. But that is true of virtually all new cars in this class.
Bare in mind though that I have Brown & Wilson speakers (B&W) on my home stereo system (Arcam CD player, custom modified 1980's Technics amplifier) and most of my previous cars have had Alpine or Pioneer HU's with infinity (Kappa)speakers, so I am used to certain sound qualities on Classic, Opera and Rock music that no standard speaker can provide.
In summary, stock speakers no worse than any other cheap car and if you are an audiophile then upgrading is necessary as it is with any new car at this level.