And it never will be at 24mb, not unless you were living right on top of the cabinet. 24mb is the theoretical max an ADSL2+ line can go, in practice this is limited by your distance from the exchange and your line conditions. Your line attenuation is accurate with your downstream rate. On the plus side you now have an excellent upload rate instead of the 128k limit you had on Go Large.
For the record Go Large: FS/128, Big Time: FS/FS. Your upload speed has increased meaning you should now be able to conduct multiple tasks better on your connection or have more people browsing.
From the Telecom Website:
Factors affecting overall connection speed:
How close your house is to the exchange or roadside cabinet
Your internal house connection and telephone wiring