It's a real shame isn't it, as when they are working right they are such a good bike and a bit different from the norm.
If they had a change of heart and had a better after sales service, they would double their sales easily!
I got this tip from Motostrada in Cockeysville, MD... presumably, they in turn got it from Aprilia.
Basically, all you do is hang a moderately heavy item (can of paint, jug of water) from the rear brake lever over night. I can verify from first-hand experience that this eliminates (for a while anyway) the mushy feeling that, over time, can creep into the rear brakes on the Tuono.