I towed a 6,000lbs featherlite with my old X5 50i. Trailer had a 200lb tongue weight and I was probably over the X5's stated max tongue weight but never had any control issues. It had air suspension in the back which leveled everything out.
I used the following brake controller and it worked well.
http://www.tekonsha.com/products/bra...RB508pViEKmVBf