Pulling TH at 7900 vs Gooseneck at 9900

I drive off road in field, so I would stay with Gooseneck vs Kingpin. But the backing up in horse trailer because the axle is further back, is something I did not think was a problem. I am staying under 25ft ground.

LEQ horse trailers axles are generally located much farther aft on the frame rails than a bumper pull or conventional GN or 5th wheel trailer.
This creates more hitch load on the truck than a conventional GN/5th wheel trailer.
So yeah your going to notice a difference especially in backing. With the axles farther aft their a bitzh to back in tight situations.......especially compared to a bumper pull trailer.

The diesel will still pull the heavier trailer with no issues.

No and I converted all my goosenecks to kingpin...much easier to hook up.

Most likely, no. My Duramax pulls 18,000 pounds (toy hauler) and it pulls great. You're half of that. I think the big difference will be you won't notice the wind as much, if at all, with the gooseneck. More stable than the BP and much easier to back up.