Well, I see I am a little behind in keeping this thread up to date...
The trailer is holding up fine. I figure it has been towed approximately 35,000 miles (mostly on-road) - we have had no issues with the structure.
To answer your question, if you like the 6 wide, changing the axle and lifting it are very easy to do. I don't know that you need to upgrade the axle - I wanted the parking brake feature of the brakes, so I had to upgrade to 10" brakes, which are only available on a 3500 lb. axle. The trailer weights around 1000 lbs, so there is a little overkill. But I do still flex the suspension, so the suspension is not so hard that it beats up the trailer. Lifting the torsion axle is simple - it is essentially a piece of channel bolted between the frame and axle.
Larger tires are great - I don't worry about towing the trailer over anything. Same as on the truck, so I only carry 1 spare tire.
I suggest you add a skid plate if you plan to take the trailer off-road. The skid plate is welded to the frame, adding both additional rigidity and weight down low, lowering the center of gravity. With the taller tires, I have dragged the skid plate a couple of times.
I wouldn't hesitate to modify the trailer.
Funny thing - I always planned on building a teardrop. We bought this one to be able to camp now - while I built one. Now, I don't think I will build one - I don't feel a need to change anything. This one works great for our use.