I did a pulley change on my 2010 Corvette Z06 which had the LS 427 motor (in addition to a lot of other mods). I would say in that case, it was worth it. For the Shelby, I would say no, and here's why. When you change pulleys, you will reduce the amount of charge put back into the battery. The GT350 stock battery is extremely weak in CCR already, so I would be very hesitant to reduce anything that recharges it. Actually, I am going to look into a Lithium battery replacement (they have made great strides lately) to see if that is a practical alternative. It would save at least 20 lbs of high up front end weight on this car. Anything to reduce sprung or unsprung weight on this car would be a gift that keeps on giving. For example, replacing the boat anchor 31 lb wheels with 21 lb true forged and strength tested wheels would equate to about 176 lbs equivalent of unsprung weight (reduction of unsprung rotational weight yields big returns).
Don't have money for that right now, still waiting on my DCTS steering wheel from the group buy months ago!
When I get some time, I will document some power changes that were quite effective..