I did not run the upper, but did do the upper upper/alto stretches in November at medium flows (~10k was the locals guess).
You can also try Cacho at Mi Tierra ( http://www.hotelmitierra.net/
) who has kayaks as well as kayaks, guides, drives, etc. Good guy, paddled with him a decent amount when down there last year. His hotel is located in Alajuela, maybe 5 miles from the airport. This may or may not be useful depending on what you transport/hotel situation is.
August is rainy season (as is may to decemberish) so who knows. Last year was drier than normal everyone kept telling me.