Measuring Character Set Conversion
The old csscan used to do this, but have you actually run DMA?  DMA does a scan of your whole database and then lists all the tables that will suffer a lossy conversion.  This is a preview before it actually goes ahead and does the conversion.  You will have a higher chance of lossy data if you go from a multi-byte character set like AL32UTF8 to a single-byte character set like WE8MSWIN1252.  There are many reasons why you would have lossy data, and part of these are due to databases being accessed by multiple applications and tools on multiple operating systems, with locale/NLS_LANG set correctly or not.  If NLS_LANG is set correctly then oracle does single-pass conversions between source and target database.  This depends on the application though.

Oracle does state that there is a performance overhead (maybe 5%) on using a multi-byte character sets, but specifies AL32UTF8 as the default as that covers all character sets world-wide.  I don't know of any view that calculates conversion times; that's part of the RDBMS.

We personally have chosen WE8MSWIN1252 on our databases for two reasons:


  
  It was going to take too long to convert from US7ASCII to AL32UTF8
  It covers most European character sets and we have separate AL32UTF8 databases in our eastern countries
  


Hope that helps!