1. With your 9m the Hyper will plane earlier (if finned correctly), with sizes 6-8m, there wont be any significant difference.
2. Straps are not your problem, you have the wrong board! Sounds like you are using wave sails? and in bump and jump conditions? If so you need an Acid or Kode/Kombat.
In a nutshell you have 3 very similar boards, keep you favourite light to medium wind board, and swap the other two for a decent bump and jump board, if thats your intent.
Although the reccomended sails for the F122 say 5.5m - 9m, you can guarantee that the extremes of both sizes will feel awefull. These sizing are foronly if you have the one board. If you have 2 boards, always size your sails for the middle of the reccomended range. For F122 that would be 8.4/7.8 - 6.5.
If you could let me know what sail types you have,and what you are trying to achieve, there are a lot of informed people here who can help more.