The thinner air at 5000' helps a lot on distance. Just look at Big 10 Inch who shot about 400 feet further (almost 4800', I was wrong above) there than they've ever shot in DE. My guess is Colossal Thunder's 3297' at sea level would have been right around 3K, give or take 50-100 feet either side.
The below pics are from the 2013 WCPC, it looks like they have since re-arranged their pumpkin box to be pushed well off the arm, allowing for a longer sling as it gives the pumpkin a "head-start" on the arm.
Colossal Thunder, Nov. 2013
CW-main axle linkage on Colossal Thunder, Nov. 2013
Colossal Thunder pumpkin box, Nov. 2013. Since pushed way off the arm on an extension for 2015.
I can't find a 2015 video on Youtube, but if you have a Facebook account, check out the .
I'll try to link this video here, should work without an account: