Right from a discussion internal reflexions (thanks to Ross Smith IV and Brian Day for their help on putting metrics down), here is the final statement.

Based on BES documentation :

http://docs.blackberry.com/en/admin/deliverables/8864/BlackBerry_Enterprise_Server_for_Microsoft_Exchange-5.0-US.pdf

We calculated that in summary based on their whitepaper for E2010:

IO Multiplication Factor: 2.0

Megacycle Multiplication Factor 1.5

Use these factors in your calculations, for example in the Exchange 2010 Storage Calculator.

Sam