That's odd. I must know 10 auto 5.0 guys and we ALL get better runs by leaving at idle. Sometimes I will 60' better leaving at 1000, BUT when I looked at my slips it hurt me in the big end. All of my better ET's are at idle. This was discussed on here in length a while back and the consensus for the best OVERALL pass with the stock converter was leaving at idle. But yes, some cars can react differently.
As for the 26" tire comment, I had forgot he was running those short tires.
1-That could cause a 3-4 shift, but with a tune that should be at like 125 mph, so I doubt it.
2-Does Mike know you run this shorter tire? As this will effect the shifts, etc for sure.
3-With a stock converter and 3:15's you shouldn't even need a tire with just basic mods.