I don't think Yeshu is based on Bar Kochba, or at least there is nothing in the GT that would lead me to believe so. Bar Kochba, at least for a time, had potential and a degree of success in driving Romans out and attempting to re-establish Jewish sovereignty. Plus, there are letters written by Bar Kochba, and coins minted in honor of the short re-establishment of a Jewish reign over the land, which is more than can be said for Yeshu. I have a hard time believing the gentiles could have "hijacked" Bar Kochba and turned him into the Yeshu figure.