So yeah, same here. Looking at my cars from this year (4 and 3/4 wheel base) vs my cars from last year (5 and 5/16" wheel base), I'm guessing (emphasis on guessing) that the wheels are getting moved back by a lot of the pros b/c, in addition to the fenders, it moves the COG back. My theory is, if you tune the car perfectly, that's going to be an advantage. More weight further back means more momentum. and speed. If you don't tune the car well, with that shorter wheel base, you might have some pretty serious wobbles though. Again, this is all a guess from someone that's far from an expert.
Last year, I had a much lighter car body, more tungsten cubes, and a COG that was pushed farther back than the year before. We put 10x more time and research into last year's car and expected a much better result. We won the pack race both years, but last years car was actually a little slower than the year before. Watching it go down the track, it's no mystery why. It was bouncing back and forth so hard, I was surprised that it didn't go flying off the track. It was a good track in good condition. Fortunately, it stayed on the track, but the experience was a valuable lesson. It's important to know your limitations. Depending on the skill level of the builder, at some point, less is more.
This year, we still don't have a decent tuning table. I'm just putting the 5kidsracing printout on a 5'table and rolling the cars on that. No matter how tight and flat we try to get that huge piece of paper, we're not getting consistent steer. I'm concerned the paper is still bubbling up just enough to alter the direction of the car. I'm looking for a piece of glass or something to put on top of it, but haven't found anything good yet. Our race is on 2/6, so we've got a little time to get it figured out.