I don't have the same problems tucking as many other folks seem to have But I have heard of them and it makes sense for different body types/ boat combos. The GreenJacket has a nicer volume distribution than the A300 so I would check it out. I am switching to one though I will always love the A300.  
  
I am biased for Astral because several of my friends work there full time and I get temp work from time to time when production is busy. I have personally hand stuffed about a hundred Greenjackets so I can tell you that they are made right by a whole team of very skilled workers right here in North Carolina. On top of making the best PFDs anywhere, they stand behind them with great customer service and that's no rumour. I may be biased but I am honest.

As for raising your seat. Very simply cut some foam to the shape and adhere it in there. I would suggest using one inch and .5 inch sheets of foam stacked so that you can tweek out the the thickness.  There will be noticable differences from 1-2 inches.  use duct tape till you figure it out.

There are pros and cons to raising your seat.(This is not a comprehensive list)

-some of the cons include making the boat less stable and making it harder to roll.
-some pros include being able to edge harder, feels a little more high performance.

I tend to think that raising the seat is good for my wave surfing and not as beneficial to my creekboating. This is just my limited experience from jacking up the seat in my playboat versus never even trying it in any of my creekers. If anybody has any experience that thinks raising the seat in your creekboat is a great idea let us know?

I hope I have helped you with my biased suggestion and my limited experience.

G