Francois- This spreadsheet calculates the correct size fins to be used with different size sails.
Straps forward will not help your upwind performance, but it may help you get accustomed to sailing a modern board. When you are comfortable with the forward, inboard positions on your 144 you should move the straps back and out for better speed and upwind performance. I.e. the straps should be where they are in the picture here:
As PG notes, for the hypersonic you can use a somewhat shorter fin than recommended; perhaps 6 cm shorter, but it works ok with normal length fins, too. When I had a hypersonic it came with 2 fins; a 34 cm and a 44 cm. I used the 44 cm fin with sails 7.5 - 9.4, and I used the 34 cm fin with sails 5.2 - 6.6. I don't know if you've sailed the hypersonic much yet, but I have to warn you that it's a very difficult and technical ride which is sensitive to where you put the mast base, how you rig the sail, what size fin you have, where the footstraps are, etc. It will take a lot of trial and error to figure out what settings work best for you.