Regarding Extended battery like "engineer"

does anyone who has used the Engineer 4KWT Extended battery know if they really help in getting better MPG. whats the Avg MPG people get with extended battery (Like the engineer one).

i have heard that with extended battery, you cannot drive above 25 miles/hr with all electric mode.....is it true ?

i want to know if its really advantage to buy Prius and add extended battery pack and pligin to it inorder to get improved mileage.

i drive avg 60 miles a day (mostly highway miles) .