There are benefits of passing your full test, the main one being the option if you do want it, you can ride any bigger bike. Also my Insurance when i had my 125 went from £400 on a provisional down to £176 with a full license.
Also if you want fuel economy their are bikes like the Kawasaki ER5, Honda CB500 and Suzuki GS500 which you will do your DAS on, which offer up to 80 mpg. Even on my 600 i get 40 mpg when i attempt to thrash it.
Honda are also bringing out the NCX 700 which returns good fuel economy.
Shop around for DAS prices. I used a company called Williams in Bolton, was 2.5 days training and test fee and that cost me £350.
I don’t think you’d need more than 3 days, if you have the road sense thats half the battle, just need to learn clutch control on a bigger bike.