RE: North Sails "One mast fits all" concept.
Might work, but if you read the "best mast" specifications from North (and basically ALL other sailmakers), your 4.7 needs a 400 cm IMCS 19 mast and your 5.3 needs a 430 cm IMCS 21 mast.
A 6.5 needs a 460 IMCS 25 mast.
8.5 m2 rigs need a 490 cm IMCS 29 mast, and sails larger than 10.0 need 520 IMCS 34 mast or 550 cm IMCS 39 mast.
As you can see, the larger the sail, the stiffer the mast required.
Adding the North CX (Carbon Extender, or any other type of extension to the top or bottom of the mast) does not make make the mast any stiffer, so the more extension your add, the poorer the perfromance of the sail (unless sails were designed on extended masts, which they are not).
Even when you have a sail that takes a 460 cm extended out to a 490 luff length (with 30 cm of a normal 46-48 cm extension under the bottom) you can be assured that the sail designer actually used that combination (460 mast with 30 cm of extension) as the basis for the design. (This is why virtually all sail designers recommend a "best" mast. The "best" mast is what they used when they designed the sail and provides the most likleyhood that the sail will perfrom for the customer in the way that it was designed.
Use a different mast, and you will surely get a different result.
Is having the best mast "critical"?
In some cases , absolutely, in others cases a mast that's "close to the right specs." may work pretty well.
If you consider that we "pay" quite a bit for the best sail designs, it makes very little sense to compromise those designs by rigging the sail on a mast that does not provide the full performance you have paid for in the sail.
Hope this helps,