How is the performance of Exchange 2007 better than Exchange 2003?

In Exchange 2003, the database read to write ratio was typically 2:1 or 66 percent reads. With Exchange 2007, the larger database cache decreases the number of reads to the database on disk causing the reads to shrink as a percentage of total I/O. 

In Exchange 2003, a transaction log for a storage group requires roughly 10 percent as many I/Os as the databases in the storage group. For example, if the database LUN is using 1000 I/Os, the log LUN would use approximately 100 I/Os. With the reduction in database reads in Exchange 2007, combined with the smaller log file size and the ability to have more storage groups, the log-to-database write ratio is roughly 1:2. For example, if the database LUN is consuming 500 write I/Os, the log LUN will consume approximately 250 write I/Os.