I think both Andrew and GVP are partially right. Andrew is right that a great chassis will make an average motor look good but if you are in a big race like the R/4 or Fall Brawl you better have a great motor if you want to make the A main. Likewise GVP is right in that you have to have at least a great motor if you hope to do well, a great chassis and a average motor just won't cut it in a big race.
And here's a fact of physics, the coefficient of static friction (i.e. not sliding) is greater than the coefficient of dynamic friction (i.e. sliding) so a car that is hooked up perfectly will come off of a turn and accelerate quicker than one that has a slight slide out of the turn even with identical motors. I've seen that the cars that have virtually no slide in a turn (in fact they look like they have a second guide!) come out of the turns like a rocket and pull me a foot or two after the finger even though our motors are even down the straight.
And let's not forget driving ability, here's a quick personal example. I was practicing my GT-12 on a tight flat track in NorCal and after a whole morning of testing and tweaking I had gotten my car to a time of 3.000 sec and felt nobody could go quicker with my car. It just so happened that Paul Gawronski was practicing next to me and confident that I had gotten the last .001 sec out of the car I asked him to take a few laps with it. After just two laps he had already taken .150 sec off of my best time, lesson learned!
Bottom line is that you need both a great chassis and a great motor, not to mention exceptional driving skill, to win the "Big One".