Josephus has probably been tampered with, but I think most scholars agree that the mentioning of a Jesus (son of god or son of carpenter) was not added by later Christians. The embellishments ("He is Christ") are certainly added, but the mentioning of a Jesus probably not.

(Besides it would have had to have been tampered with before 324AD, for Eusebius quoted the entire relevant passage of Josephus then in its 'current' form)

I pretty much agree with this
This also states it pretty nicely.

This is much more of a technical matter than anything else, Christians will not forsake their faith even if somehow it would be conclusively proven that Jesus was all just one big pratical joke. And even atheists/agnostics would gain little, for it probably wouldn't convince them any more or any less on the plausibility of God.