I see what you guys are saying by disagreeing with the bigger spinnaker. I would have thought the same thing having sailed F18s and I-20s.
However, last year (summer 2005) we had 4 or 5 I-20s at a regatta in Massachussets and 3 6.0s. It was my first regatta on the 6.0 and first time sailing it with the snuffer set up. The guy I bought my boat from who was within the top 3 best 6.0 sailors in New England at it's peak was also racing a much newer 6.0. On the first day in heavier air the previous owner of my boat beat me every race. He ended up ripping his spinnaker towards the end and was only able to find a I-20 spinnaker as a replacement for the 2nd and 3rd day. The 2nd and 3rd day were light to moderate and I beat him in all but one race and by large distances. It was clear that downwind I was faster with the bigger spinnaker, I was even passing the I-20s downwind.
This year at the statue it was a broad/ high beam reach after the start. The I-20s were able to carry their spinnakers much higher while I had to sail lower. My spinnaker was extremely shot and certainly not as fast as the new ones the I-20s were carrying. As the I-20s went high, I went as high as I could, double trapped and no where near as high as the I-20s. When we got to the statue some 18 miles later I was right on the heels of the 2 lead I-20s. Although I am sure I had to work harder as we were doubletrapped and manhandling the spinnaker, but that is always going to be the case when it comes to 6.0s and the I-20.
You might have something upwind though, I can see how the jib might negativley affect the main especially if the trim was wrong. I know when when we raced the I-20s in lighter to moderate stuff it seemed to me the 6.0 was a little slower and in heavier even slower. I can't say whether that was the jib or not.
When it comes down to it, the 6.0 is 20+ lbs heavier and needs power from somewhere.