I'd rather not file the splines. If the sprocket is case hardened or otherwise hard-plated, filing will remove that hard layer, and wear will be accelerated due to load at the sprocket/freehub body.

I like the idea of going to a 9-speed freehub body. I still have to use only 8-speeds since I have 8-speed Ergo levers. Wasn't there a conversion kit to upgrade the 8's to 9's, in the right-hand Ergolever?