It's not so much that the 320 is only limited to 50% of spoilers with the AP on, but you get the extra spoilers over the 319/321 and 320 with AP on. 50% of 320 spoilers are equivalent to what you get with the other mini-buses. Last ferry flight I did we decided to see how much you get and believe me it is impressive! Turned the AP off, selected 300kts in an open decent and full speed brakes… Was a pretty impressive decent anyhow!