I don't know about anyone else, but I have not seen a problem with the chassis becoming tweaked because of motor heat??? I give my chassis the once over before a race. This includes placing it on my Tweak Station. I find very little variance if any race day after race day.
Doc Martin: I can't address your question directly since I never owned the Worlds chassis. I do, however, own the Pro and I noticed a significant difference between the Pro and the Special. It would be difficult for me to isolate the causes for the differences, but I find the Special much more forgiving to minor mistakes. Also, using dual oneways makes my Special much easier to be agressive in the infield than I can be with my Pro.
When I first got the Special, I tried to tune it the same way that I tuned my Pro, (even running it in 4WD) and it didn't work. The Special runs with a softer setup on the same track. This suggest to me that there must be differences between the two chassis.
Whether it's worth the expense I couldn't say. You'll have to make that decision. I would suggest trying someone's Special if you can.