My experience with "alternate" masts has always been that you can achieve some differences in the handling and performance of the rig by using a mast that's either
shorter/softer (460cm IMCS 24-26 in a sail designed for a 490cm IMCS 28-30 mast) or
stiffer/longer (490 IMCS 28-30 in a sail designed for a 460 IMCS 24-26).
If you are a lightwieght sailor, often the softer/shorter mast will give the sail the ability to twist off sooner and at less pressure so you can handle gusts more easily.
Sometimes, by very careful rigging (not "by the numbers) you can get a bit more draft and make the sail a little more powerful.
Conversely, if you are a heavier sailor, you can use a stiffer/longer mast and get the opposite effects.
More tension in the upper panels of the sail will put more power higher up in the sail and if you are big/stong enough to handle this, the sail can be rigged more powerfully and not twist off as much, and at a higher pressure.
The Sailworks Hucker sails are the most "tuneable" this way.
Take the 5.6 m2 Hucker and put the recommended std. dia. 430 IMCS 21-23 100% carbon race mast in it and you get the designed power and twist. The Huckers have more power up high and this is most evident on the 100% 430 Race Std. dia. mast.
Put in a 430 cm IMCS 21-23 Sailworks Backbone RDM and you get less power up in the top of the sail, and a deeper draft (due to the luff sleeve sizing around the dia. of the mast) down lower in the sail when it is fully loaded.
Put in a 460 cm Sailworks Backbone IMCS 24-26 RDM and you get a full on slalom tune (BP has used this combo with lots of success in the Gorge Cup Races) with a flatter head and more power down lower, but still fully tuneable with the adj. outhaul and on the beach downhaul adjustments.
So, give the softer 460 cm mast a try. You might like it. Do not go by what it looks like laying on the beach. Take it out and get it fully loaded up. That's the only way you can tell if it's better or not.
I'd guess that since Gaastra recommends the longer stiffer 490 IMCS 28-30 mast, you are going to see the top of your sail dump off at much lower downhaul tension and it will dump off alot more than it will on the 490 mast.
Whether this is good or bad...... only you can make that judgement out on the water.
Hope this helps,