RE: Gybing a 150 to 175 liter board.
Perhaps what you are experiencing has more to do with the techniques you've learned to gybe, than it does with the board you are gybing on.
Nearly all boards have a "sweet spot" where they do nice pivot and flare gybes (the position of the sweet spot changes with sailor weight however).
When you get to doing fully carving gybes, the technique must change as you are carrying more speed all the way through your gybes.
Your suggestion that an 8.4 m2 and 6.0 m2 are "large" sails is perhaps a bit antiquated. 8.5 and larger sails are "large" sails; 6.5-8.5 are "medium size" sails and 6.0 and smaller are getting into the "small" sail range.
So, to get good results on a more modern board, you are going to have to modify your techniques a bit to suit the board.
This was true back when the Calypso and Samba were the current design, and it has not changed significantly since.
You need to go out and discover where to stand on the board to get the best stability, how much to "rail" the board, how much to rake the rig over the side, and develop the timing that's specific to the board.
This is logical since each board has different rocker and rail characteristics so you as the sailor need to develop skills that suit these characteristics.
With the newer wider boards like the GO and Carve, you do not need to stop the board and sink the tail to gybe.
Just find the position on the board that keeps the board at the same pitch angle of attack (front to back trim angle here) that keeps the board planing through your gybe and you can do fully planing gybes in windspeeds that you could only pivot or flare gybe your older Calypso and Samba in. The added width and further back volume distribution support planing gybes in lighter wind conditions.
Certainly you can flare or pivot gybe the GO and Carve, but since they were designed more for light wind planing gybes, they will become unstable when you pop the nose up high.
So perhaps you need to change both the type of gybes you are doing as well as the technique to suit wider boards with more volume near the back.
Hope this helps,