Hi ERay,
I've been waiting for an answer to your question along with you. I know that presently there's a peening spec conference going on in California and that might delay a more qualified answer. I can give you my thought in the meanwhile.
I think using plastic media is a perfect choice to blast paint off. It is non-abrasive so the surface should not be damaged. It's also a very low mass media and shouldn't have a negative effect on the residual compressive stress layer.
If you wanted to be sure you could invest in some Aero-Almen strips. They're used for testing the effect of blast cleaning on aerospace/aviation parts.
I recommend reading this document
http://www.electronics-inc.com/uploads/paintstripmanual.pdf giving special attention to section 6.3.
I hope this helps out.
