I do not have any personal experience with the 10.0 Gaastra Swift, but my educated
guess would be that you would get significanlty more top twist (the sail is much looser and the looseness will extend in much closer to the mast) and a flatter mid section if you use the softer 490 mast in place of a stiffer IMCS 32-34 520 mast.
I've done a bit of testing using stiffer/softer tops on shorter masts to "tighten up" the top section of several sails to give more shape and power and that has been my experience.
So, if you put more extension in the bottom, it does nothing to stiffen the top section of the mast.
So, more twist/ less power up in the top panels of your sail.
Yes, you can use less downhaul, and do some "creative rigging" and you might like the results, but the Swift has 2 cams, and you need to get the downhaul pretty near the design optimum if you want the cams to fit/rotate correctly.
Give it a try if you like, but also remember that if you get too much extension in the bottom of the sail (sounds like at least 36 cm for this sail on a 490 cm mast) you begin to run the risk that you will "point load" the mast at the end of the extension and the mast could break at this point. I've seen over extended masts break in this manner a significant number of times.
Also, if you can borrow a good 100% 520 mast, be sure to give that mast a try in the
I think you will find it has more power, and the overall balance will be better.
Hope this helps,