The major difference between a no cam sail and a multi-cambered sail is sail stability when sailing overpowered.
Full-on cambered slalom or formula sails are sailed well- to over-powered. Cambers help a lot to maintain foil stability under load and power within the first 1/3 draft of the sail.
With no cam sails such as wave or freemove sails, draft and foil stability do wander a lot back and forth while being sailed overpowered and you do have to counter such variation with more traction/force with your front or back hands. You will have to fight more with your sail, loose more energy, control and speed. However, a no cam freerace sail such as the Severne NCX is well known to be pretty stable while being sailed overpowered, so it depends on which type of no cam sails we are talking about.
In sum, a cambered sail is used in more wind than a no cam sail of same surface and is more stable, more control-oriented and faster while sailing overpowered. Cambered sails are heavier, pricier, more sensitive to mast requirement (especially bend curve) and require more tuning skills than no cam sails.
So to answer your question about 5.5 versus 6.5 m2 sail surface, board type choice is critical as well. It is always best to match the board type with same sail type, e.g., a wave sail with a wave board or a slalom sail on a speed/slalom board. I'm overpowered with a camber 6.6 sail in 20 knots wind whereas I'm barely well powered with a no cam wave 5.4 sail in same wind.
It's very easy to answer your question about large no cam sail versus small cam sail comparison as following:
1) I have 2 combos that I'm using in the same 12-18 knots wind range.
a) For flat water, I'm using a multi cam Code Red 7.7 m2 sail on an iSonic53 speed board + 30 cm slalom fin.
b) For heavy choppy/wavy water, I'm using a no cam NP RAF Jet 6.9 m2 sail on a Kombat 86 freewave board + 30 cm freeride fin.
The first combo is way faster and powerfull than the second combo, no question. The second combo is just more playfull and easy-going in rough water for bump and jump.
2) So, if I use a cam 6.6 sail with the speed board in 12-18 knots, I'm clearly under to well powered and the second combo is therefore faster and more powerfull most of the time. So, a small cam sail is not always better than a larger no cam sail with the same wind force.
The bottom line and take home message is as follows:
Cam sail are used in more wind and in a wider wind range than no cam sail of same surface.