RE: the toes up thing
I can't prioritize for you, but if you want to get the most from your 6.6 m2 rig, I'd think about getting a medium extension.
Right now it sounds like you are going up 23 cm (460 cm + 23 cm = 483 cm) on your 42 cm extension which is only slightly more than half way.
Since your extension has probably another 12-14 cm above the top setting (so there adequate extension up the mast at the highest setting) you are getting about 30 cm up inside your mast.
So, the lowest 30 cm (approx.) of your mast is not able to bend as it's "reinforced" by the extension inside.
Getting a shorter extension might improve the low end power of your rig.
Since you don't have any sails that need a taller extension at the moment, and seem to be looking for some smaller rigs, I'd suggest the medium extension be moved to the top of your list.
Keep the tall extension, because when you get a 7.0-8.0 m2 rig, that goes on a 460 mast, you will need the tall extension.
The same applies to getting a smaller sail that needs a 430 cm mast.
430 cm masts are IMCS 19-23, and are significantly "softer" than your 460 cm IMCS 24-26.
Yes, you can extend the head cap on most sails under 6.0 m2, and use them on a longer mast, but the mast being 'too stiff" will not allow you to get the sail to rig as the designer intended.
You can play around with downhaul and outhaul "tuning" when using a mast with the wrong bend characteristics, and sometimes get the sail to actually work decently, but you probably cannot achieve the full performance, and tuning range.
At some point, you will have one of each size extensions (stubby, short, medium, and tall), and one of each size masts (400/430/460/490) so everything will rig and tune to give you the full performance.
Hope this helps,