According to this site, peak yield on a hybrid fission/fusion bomb to date is around 5 kt yield per kg of bomb (including casing, detonators, controls -- the whole package. So we need to increase kinetic energy by a factor of around 5000 to match the increase in yield. KE goes as V squared, so we'll want to go sqrt(5000) times faster -- approximately 70. So you get KE yield equivalent to the same mass of high-efficiency fusion bomb at around 200 km/sec. That's not all that fast, in a game with drives that can accelerate at multiple gees for days at a time. For comparison, it's 4 times Mercury's mean orbital velocity.