Yes, a double-bubble/higher screen is a must for mileage work, and they really do make the difference.
I bought mine from Busters Accessories - a light-smoked version which has a black panel to hide all the gubbins behind the clocks. Cost was 39.99
As for the screws ... take the top fairing off! It's 2 bolts for the mirrors, 2 bolts in front of the air tubes and 2 dzus fasteners each side - it can't get any easier!!
Pics here:
http://www.horrible.demon.co.uk/bikes/748.htm