Your amp totally outclasses the cabinet you own. A better solution may be to upgrade the drivers in the cab you already own.
In my experience, if you want really good tone, you have to invest in a cabinet that will not flex at high SPLs as well as have high resolution, good construction and good parts quality. I have found, ultimately, that I like a Rivera 4x12 top an old Hiwatt 4x12 bottom for my purposes.
Being that you play in a thrash band, you may want to go with a Marshall 1960 cab, or put Celestions in your current cabinet. You can also sell off the Behringer and pool together whatever money you were going to spend into getting one high quality cab. Even used, you will get much better performance.
Some used, high quality cabs you should be on the lookout for:
Mesa
Rivera
Hiwatt
Orange
Laney (especially really old ones loaded with Fane drivers)
Marshall
Hope this helps.