Yes, the longer (48 cm NP carbon) extension will allow you to go longer on the mast and this will allow you to use larger sails.....
Unless you are racing and using 100% carbon masts on huge sails (you aren't anywhere near that with only a 7.8 m2 rig, but even an 8.5 m2 rig isn't really all that large by todays standards) I'm not sure if there's a real reason to get an expensive carbon extension.
I use aluminum extensions that weigh an ounce or three more than carbon extensions, but I've never had an aluminum allow extension "snap off" the way carbon extensions sometimes do.
The aluminum will bend or deform long before you "break it off".
So, unless you are looking to "hyper extend" your 460 mast, I think it's better, safer and perhaps almost as cost effective to use the 40-42 cm extension you have and get a longer/stiffer 490 mast if you need to go beyond 500 cm.
Remember, you really want to use (or at least try) the recommended mast (size/length and stiffness/bend characteristics) if you get a larger (8.0 m2 to about 9.5 m2) sail.
The larger sail really will "need" the additional stiffness.
460 cm masts are in the range of MCS/IMCS 24-26 (constant curve or CC) whereas longer 490 cm masts are IMCS 28-30 (which is still a constant curve, just stiffer over the entire length of the mast.
If you try to "hyper-extend" a 460cm IMCS 24-26 mast to use it in a sail that was designed for a 490 cm IMCS 28-30 mast, you will not get the correct profile as the top of the sail will "dump off" long before you get the correct tension in the lower panels because the overall bend characteristics are simply not stiff enough.
Adding extensions to the bottom of a mast does not really change it's overal bend characteristics very much, if at all.
Hope this helps,