I'm not sure that the iSonic 131 can work well with a 6.0m rig as previously I was considering a 101 to complement my 5.8m and I was told that this board was too big and peaked more with sails around 7.0m. Therefore, a board 3 sizes up probably wont work well at all with a 6.0m.
However, that being said it would probably still be sailable.
As for early planing, width and subsequent fin size plays a big role in early planning. The iSonic 131 is 85cm wide whereas the Futura 141 is only 80.5cm wide.
Therefore, on the iSonic you should be able to take a bigger fin/sail and get better early planing performance.
I've also read that Slalom boards, as opposed to freeride, have a cleaner release making pumping more efficient and increasing early planing ability.
JP Super-X 106l, North F8 Ram: 5.8m,