Nah, it ain't me. I used to have a FA 357 (353) but it wasn't a convertible. I'd like to hear others opinions to your question, but I also think it would be more of a question of the merits of cartridge design rather than a matter of the 9mm being a accurate round. So, venture to say, it would be less accurate if you were using slower burning powders and going heavy.
There was a guy locally that was selling a short FA Casull with an extra cylinder in 45ACP and I had the same general feelings. Honestly, it just felt too unconventional for me. The selling feature for me would have been that factory ammo would have been a little cheaper if I didn't feel like taking the time to reload.
Sorry I couldn't have been more helpful.