Funnily enough I have been working out density altitudes this week. I'm involved with the F1 air racing crowd and we had two races last month, one in Tunisia and one in Spain. It was very hot in Spain, about 40 degrees, so when giving out the results, I calculated the density altitude for the Spanish race this year vs the Spanish race last year. It was 4100 feet this year vs 2000 feet the year before.
There has long been a debate about whether the guys at Reno fly faster because they are in thinner air, or slower because their engines don't get as much. The results for Spain showed a 3 mph gain per 1000' increase in altitude. This only seemed to apply to the cleaner airframes, with little to be gained by the stock slab wing Cassutts.
This would put the gold race winner at Reno at about 264 mph total race speed(average speed over 8 laps). I'm looking forward to finding out if this prediction is accurate. The current single lap course record is 267 mph.