The only reference I found was from Wikipedia, under the Ju87B paragraph. It actually says a loss of 10-20mph, (about 15 knots) The reference there points to this book:
Griehl, Manfred. Junker Ju 87 Stuka. London/Stuttgart: Airlife Publishing/Motorbuch, 2001
I especially like the paragraph on "diving procedure". Very interesting. Here is a link to that site, in case it's harder to find in Europe, etc.
http://en.wikipedia.org/wiki/Junkers_Ju_87