There are so many factors involved that I don't believe there is a universal answer. There is a personal preference part. Garret and I have an acquaintance who can absolutely smoke us in the sand on small bikes. Yet he has an AT which he won't ride off-road because he isn't "comfortable" on it and is concerned he's going to injure himself. Then there is the less subjective part. Putting the same amount of gear on a big bike has less of an impact on that bike's performance than putting that same amount of gear on a smaller bike. I believe a large bike is also less fatiguing than a small bike on easier terrain - like that which constitutes most of the TAT. I distinctly remember being very uncomfortable riding my loaded 450 on the freeway at 70 MPH whereas it's hard not to go 80 or 90 with the AT under similar conditions. I think the KTM 500 EXCF would be a fine bike to do this on - especially if you are comfortable with it. I'd like to do the whole coast-to-coast thing again. Looks like I might go to the KTM Rally in Crested Butte next month by way of Andrews to Gunnison on the TAT route with a buddy. That'd give me a chance to see what the first half of the route is like in September. You might consider starting in Daytona Beach and going through Ocala National Forest. That'd give you a closer start to your origin plus a true coast-to-coast. The only reason we didn't do that this year was because I wanted to do the same route I rode last year.