Like Ian, I too use the Sigma 70-300MM with my Canon 350D.
And like Dave, you notice the heat haze at longer distances but that would not change even with a different lens; it is all due to the weather.
Here at Southampton, you can hear the "Yippee!" when the wind is coming from the north or north east. Usually, it comes from the south west and brings a lot of haze and moisture in the air.
The northerly wind is nice, cold and clear! (also rare!)
Kelvin
(I also may have won the lottery with a job offer abroad!)