cutting it in half... would still require a long fuckin time
For example, when New Horizons launched back in 2006, it took the spacecraft 9.5 years to finally reach Pluto. Pulsar Fusion claims that it could cut that travel time in half.
Article is capped at 18 views/day so can't see numbers.
But theoretical cap of energy would be something like E_kin = (\gamma -1)mc².
Without knowing anything about the mission or engine, a 50 kg probe at a velocity of .9 c means an energy requirement of about 1,0e19 J.
Fusion of H2 to H3 yields about 340e9 J/g meaning we need about 3 million kg of fuel at 100% conversion rate, or a third if we manage He3 reaction.
Realistically heating, engine efficiency, deceleration, vibrational damping and such would probably lower efficiency to at most 40% and we end up at 8 million kg of fuel to propel a 50 kg payload (not counting the fuel mass).
the mass increase due to relativistic effects is really really small, significantly smaller than the weight loss due to fuel consumption, which btw is described by the Rocket Equation.