My tractor is well over 6500 with the loader and bush hog. For 95% of what I do, any additional weight would give me a pig in a poke. /forums/images/graemlins/shocked.gif That is why I haven't considered loading the tires or adding any additional weight. Also, having used higher hp tractors that do weigh more, I'd have to say that you would be severely underpowered if you had a lot more weight. So what if you can move 15,000 pounds at 1 mph. When disking and plowing you need some pretty good speed to get the job done right. Both gear drive and hydro tractors in the 50 to 55 hp range will flat run out of oomph of you load them up to over 10,000 pounds. I'd say that 65 hp would be marginal for most work with that much weight. For that weight, I prefer using tractors that have over 90 hp.
In the swampy areas, heavy weight will result in basically a giant anchor. /forums/images/graemlins/tongue.gif With no hard bottom, the more weight, the more you sink. Have you ever seen a dozer with "swamper tracks"? I had one. They are really wide; like 20" or so. That is to spread out the weight so you don't end up with a giant anchor as well. Also, the largest tractors around here use triples on each corner. The whole idea is to prevent the weight from compacting the soil and from sinking and getting stuck in the spring and fall.
I couldn't imagine using a heavy 2 wheel drive tractor's loader in anything but the most solid of soil. We have a lot of mud where I live. A local sportsman's club has a 2wd tractor with a loader. If they want to use the loader anytime but in the summer, they have to bring another tractor or a 4x4 truck along to help it back up once the bucket is full. It's really heavy built and probably only has around 30 hp. After having it a few years, they don't even bother to get it out if the ground is soft at all. And, it even has ag tires. My tractor is 4X4 and weighs almost too much as far as I'm concerned and it is under 7000 pounds. I have to agree; unless you have a specific task, I see the additional weight more as a detriment than an advantage. If I need more weight, I can easily add it. I just don't need it.