Typically ducts reduce in size as they extend out to increase flow however this is something that cannot be completely overcome or even from a room located directly next to the main unit.
The only real options other than getting a new unit/overhaul to ducts currently in place is to go the cheap route and close vents in other rooms enough to where all vents get as close to even flow as possible. Flow alone will not solve the issue as the temp of the output by the time it reaches you might be diminished as well. In that case it might be best to have actually less flow the closer to the source and increased further out to help compensate for the lose in temp transfer to the end of the duct work.
Also an additional return could be install in your room or a box fan as someone else suggested could be a cheap way to get air flowing from cooler parts of the home into your or blown the hotter air out of your room into the rest of the home.
My Dad has worked in AC sales for about 15 years and these are the things he'd suggest.
He's also said the Mitsubishi AC's are pretty impressive though only a good choice for specific situations.
http://www.mitsubishicomfort.com/en/consumer/lifestyle-benefits
I have a "bonus" room above my garage in my house. It's the only 2nd floor area of the house that is otherwise a slab foundation ranch, I have just about all the same issues you've described.
The solution I went with was in the summer I use a "spot cooler" portable AC unit that sits in the room, has wheels and has a exhaust vent that connects to the window. I went with this option for a few reasons. One reason was this window faces the community on the front of my house and I didn't want eye sore of it being something that everyone can't help but see. Also, with it being somewhat easy to move around, I can move it to any other room in my house quickly. This really became handy last summer at the peak heat of the year when my main AC unit went out and I was waiting for a repair man to come out.
I paid about $300 for it from Costco when it was on sale.
In the winter it's pretty simple to maintain since its cold outside and with the room being the only upstairs room and that heat rises, it gets way to warm in there. But all I have to do is crack open the window and the problem is solved.
