If money is no object, LeadTools is the traditional "go to" library for image processing. That being said, my first inclination would be to code it up using stock .NET GDI+ calls and then do some testing. It is likely this solution would be performant enough, but if not, you'll have a baseline from which you can compare other libraries and solutions. Anything involving spawning a command line tool will entail creating a separate process for each image, which could negate the benefit of going to unmanaged code.
