RE: the toes up thing
The reason that the WS hardware guys make stubby (appox. 4" long inside the mast), short (approx. 15 cm of adjustment), medium (approx. 25-28 cm of adjustment), and tall (approx. 45-48 cm of adjustment) is so that you do not have more than about 15 cm of extension up inside your mast.
If you put a tall extension set at < 25 cm inside your mast, you end up "stiffening" the bottom of the mast so it cannot bend the way it needs to because it has a stiff extension inside it, which will not allow the mast to bend all the way down to near the base.
This can cause some real problems with sails that have a low foot batten as the batten won't rotate correctly because the mast is too stiff. It can also take alot of shape out of the bottom of some sails.
If you are using a 48 cm extension on your 6.6 m2 rig, with a 460 mast, you really need to get either a medium or short extension (depends on the luff length of your sail) to get the best shape and batten rotation in the lower part of your sail.
Can you use a 42 cm extension on your 460 cm mast to make it into a 490?
If your sail is designed for a 490 luff length on a 460 cm mast with 30 cm of extension, then yes you can do this very nicely as it's the way the sail was designed. A 460 cm mast has a 12% bend ratio, and is rated IMCS 24-26.
If your sail is designed for more that 508 cm luff length, or is designed for a 490 cm IMCS 28-30 mast, then no your mast will be far too soft and you won't get the correct tension and shaping of the panels in the sail as the sail was designed to work on a much stiffer mast (490 cm IMCS 28-30).
Hope this helps,