The T-6s that show up to compete at Reno are almost always souped up versions, with modifications to the engines, the weight of the aircraft and drag reduction applications like the silver tape covering any seams. Lohmar's comment-- "Note the gear well seals visible in this picture-- that is an example of drag reduction not found on a stock T-6.  That said, the T-6 class is a “Stock” class.  We are allowed to do some minor mods to the engine, reduce the weight, and minimize the drag, but we can not do any big changes to the aircraft (i.e. we can not use a different engine, clip the wings, cut down the canopy, etc. as the Unlimiteds do).  Basically we have to have an aircraft that is “Street Legal”.