Re: 181mm vs. 185mm
Been running my 185mm pulley since Feb 2008 - I also installed a larger HE around the same time. My dyno runs (at sea level) right after the install showed I was running rich - I had the SRT tuned at that time to optimum performance with the stock fuel injectors, fuel rail and fuel pump.
The only issue I had was the smaller idler pulley that came with the kit gave way (along with the OEM belt) around 5K miles after the install.
I re-installed the larger OEM idler pulley and went with a larger Gates Belt and I've never had a problem since. (45K miles)
I don't race but my car has been subjected to the Texas Mile, high speed desert runs (+160 mph) and hard core canyon runs (hard on the gas / hard on the brakes).
I think the 185 is great for highway driving as it gives a huge shot of instant boost to blow by cars and have some fun in the curves.
Can't rate it for the 1/4 mile racing.