OK, that's the info we need to determine the sail range for your Speed Slider.
Here are all the specifications for your board:
The suggested sail sizes are 6.5 m2 to 11.0 m2.
So, your 8.5m2 Retro is right in the middle of this range and this would seem to be
As they suggest, sails smaller than 6.5m2 are getting out side the 150 liter Speed Slider's range, so for smaller sails (and higher winds/bigger chop) you would really want
a smaller board.
Using your 8.5m2 Retro as the largest sail in your quiver, getting a 7.0 Retro and a
5.5 or 6.0 Retro would give you the best quiver spacing.
If you are going to be sailing at Fred Howard and Sunset Beach (very flat water, even when it's windy) you could easily use a 5.5 m2 on your current 150 liter Speed Slider.
If you aren't getting any "spin out" on your 34 cm weed fin with the 8.5, then you probably don't need a larger weed fin.
I've never tried the "boom end floats" so I can't comment, but I know lot's of sailors who have tried them and they seem to work well.
Where are you sailing that you need to do deep water water starts? I thought you could touch bottom almost everywehere at Fred Howard and Sunset?
Hope this helps,