Waste heat management for 200MW system in space is firmly outside the realm of present-day technology.
It really depends on the mass budget they have for that speed for if anything new needs to be discovered.
Funny - in a lot of other technology sectors, that would count as pretty good efficiency.
Now, since you you can be boiling pure hydrogen rather than the water that comes out of a chemical rocket you can make the propellant faster. But you're limited in temperature by what your reactor can take without melting so that only buys you a power of 2 or so in efficiency.
I wrote a series about how the different types of rockets compare starting here: http://hopefullyintersting.blogspot.com/2015/03/rockets-some...