I can't answer your direct question about the Cimarron. However a GN 3H Exiss Event with an 18' box weighs ~4900#, a two horse version is ~300# lighter. The weights you quoted were pretty much in the ball park.

The big differences in towing the different trailers will be based on your traveled terrain, and the axle placements on the trailers. Three hundred additional pounds being pulled down the road is unnoticeable, unless you're pulling the Million Dollar road. If the axles are placed under the center stall area instead the extreme rear of the trailer, less tongue weight will be carried on the truck, and it will pull better. A different axle placement of a foot or so, can make a difference in the pin weight.

An 18' box GN is very easy to pull, my wife and daughter tow one most of the summer unassisted. I would highly recommend the 3H over a 2H for the additional storage and saleability. The additional weight and towing length are nominal, and not noticeable once you are used to a GN.

When you first pull a GN, your initial impression is how smoothly the trailer tows. Cross winds and truck wakes have almost no effect. Your truck will corner better, and the steering is more secure. Most people are more relaxed drivers whilst towing a GN, it's an easy progression once you have some experience towing a BP.

If your Cimarron is a wide trailer, you may have to get aux mirrors and its additional width may take a while to get use to. Don't fear the change to a GN. You will welcome and enjoy the differences.

We have a 2006 Cimarron and have been extremely pleased with it.  Some trailers from 2004 and 2005 have had some problems with the fiberglass roof.  Ours has not had any issues.  The weight is about right for a bare dressing room.

We went from a bumper pull to a gooseneck.  I would never go back as the gooseneck is much more stable than the bumper pull in the wind or when around tractor trailers.  The only drawback is that a gooseneck turns shorter whereas the bumper pull tends to follow the tow vehicle.

Regardless of what you purchase, make sure that you have more than enough truck to pull the trailer and that the trailer will set level with your truck.  With our 4X4 F350 we had our trailer built with the axels 2 blocked inches and 2 inches removed from the gooseneck so that the trailer would set level.

Here are some various axle placements located on 3H GN trailers. Notice that the Merhow and Shadow trailers have the center of the axles supporting the last stall and the butt end of the middle stall.

Merhow

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=369968

Shadow

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=371590

The KB has the last stall, the center stall, and the butt end of the forward stall supported by the axles

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=369435

The Exiss has the last stall, most of the middle stall, and part of the butt end of the forward stall supported by the axles. 

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=371503

The Cimarron has the last stall and mostly the butt end of the center stall supported by the axles

http://www.horsetrailerworld.com/home/trailerdetail.asp?ID=368339

Sure agree to axle placement. Very important with a marginal tow vehicle, but less so with a tow vehicle that has a lot of extra capacity. 

Also remember that horse and cargo placement can be equally important. If just hauling 1 or 2 in a 3/4 horse can sure make a difference. You will have to play around with all that a few times to see what works best for your conditions.

Good luck!

My neighbour has a Tundra, which tows an old steel 2H BP with no problem. IMHO I would think that any number of 18' 3H trailers that weigh in the mid 4Ks, can be carried and pulled with your present truck. I don't remember reading how many horses you will routinely haul, but one or two can be loaded into the rear most stalls and not affect the pin weight very much. Carrying three horses may over load your Toy's rear axle. With two horses and some gear, you'll be grossing out at about 7500#, less than your truck's max towing capacity.

If the truck settles, additional suspension components can be added; IE air bags, air shocks or Timbrens. I don't know all the particular OEM specs, but you may need to upgrade the tires.

I would think that this type of trailer would work with your present truck, if you don't overload the tack area and carry a max of two horses.