I cannot fully answer your question as I have not sailed the largest Futura yet.
My guess would be that the FF 138 and F-Type 138 (which I have sailed extensively but no longer have to make a direct comparison) will get on plane a little earlier due to the difference in width.
The FF/FT 138's are 96 cm wide.
The Futura 155 is 85 cm wide.
I have sailed the Futura 133 and from that experience, I would expect the Futura 155 to be a better jiber and handle chop better than the FF/FT 138's.
So, I suspect you are correct, and I will give you an update as soon as I sail the Futura 155 (it just came out of the repair shop yesterday due to some minor damage inficted by the board tests earlier this year.)
Hope this helps,