If you want to use REX but not get such a hit, try installing the lower resolution textures.  I use the 512 ones for payware aircraft and the 4096 for freeware.  Those 4096 textures are going to cost you big when it comes to performance.  Make sure that you have put a check mark in the Use DXT5 compression no matter what texture set you are using.

Also with UTX, I disable the night lighting, borth roads and intersections.  Also the newest version fo UTX adds moving cars and they are a huge peformance drain.  Go to Other Terrain Features and disable railroads, highway interchanges and glaciers.  Then go to Road Surfaces and disable Moving Traffic for...  I also turn off all traffic in FSX.  This allows me to run great REX clouds and ground textures and still be able to fly the aircraft (except maybe this one, don't know  yet ;D).

If you have a higher performance system you can turn some of these back on.

I also go to Ultimate Terrain, Change Road Surface Colors and uncheck all entries that say Add Auto Imprint.  This will take all the static cars off the roads.

Some of these can be done in UTX USA and will automatically apply to all other UTX areas and some you need to go into settings for each UTX area to change.

These suggestions all from NickN.  Go here for many, many more-->  http://www.simforums.com/forums/forum_posts.asp?TID=29041

When flying payware I also personally use Low water 1.X to give me some more CPU cycles to work with.

These changes should give you some more frames to work with when flying this aircraft.

Good luck.