I did it a little while back and never really used it apart from testing to see if it worked (which it does and I didn't get charged)
I was going to revert back to the original but never got round to it. Don't think it will cause you any problems though, if I have any problems with the phone, I'm going to Apple, not Softbank, so I'm guessing they won't take too much notice of the carrier file.
As for reversing it, I haven't looked into it at all but just had a quick look at the carrier file and it shouldn't be too hard to change it to the original settings.
Either that or someone out there with a jailbroken softbank iphone will should have access to the original .ipcc file. I'm guessing you'll just need to do the same process but with that file. 🙂