RAW, droids are subject to the same restrictions as non-droids when it comes to raising characteristics: the only way they can be raised following creation is via talents, or cybernetics.
However, the droid PC in my party is wondering why he is not able to enhance his droid's characteristics in conventional ways (using XP), but not using cybernetics. His example for "soft" characteristics like Willpower or Presence is that these could occur via software upgrades. And for "hard" characteristics like Brawn or Agility, via the installation of superior hardware. He is upset that he's had to spend more XP than all the other PCs in the party to raise his characteristics, and is wondering why this capacity to build "from the ground up" does not extend pass the creation phase.
I do not agree, and wanted to run my counter-arguments by everyone to see if they pass muster.
1) The proposal could essentially mean there is no upper limit to any characteristic, barring XP. (The Meta Reason)
Obviously this is not how characteristics are intended to work, and the artificial nature of the droid notwithstanding, this is also not how any species treats innate qualities.
2) Droids still operate within design parameters, just like meatbags. (The Fluff Reason)
There's a reason a Toyota Tercel doesn't have the strength of an M1 Abrams Battle Tank: it can't physically hold a Honeywell AGT1500 turbine engine. Assuming all droids are built for specific functions, their chassis cannot conceivably fit an unlimited number of upgrades, characteristic or otherwise.
Taken together: the character creation stage for a Droid represents the "design" phase of that droid's type. Other than cybernetics, this is a hard ceiling: the droid's design parameters do not allow for unlimited upgrades/improvements.