Help needed understanding turbojets and airplanes

Nicolas Charmont equipped a Cri-Cri homebuilt with two AMT Olympus
turbojets each producing 51 lbf of thrust that were originally
designed for model airplanes and it flies at 130 mph. I have since
read about another company, FTT (www.fttinc.com) that has an engine
for UAV's producing 37 lbf of thrust, with models producing 100 and
250 lbf under development. This made me wonder how much plane could
one usefully fly using 1 or 2 of the 250's. In comparison, I know the
VLJ market uses the Williams FJ22 (or similar engines) producing
550-700 lbf or more of thrust.

Say, for example, one took a 250 lbf engine and mounted it on top a
C172 wing. Ignoring such changes as weights & balance, etc (in other
words, I am just looking at applying the thrust to the airframe with a
normal fuel and passenger load) how would the performance of this
plane compare to a standard C172 with a 180 hp O-360 piston engine?
Would it get off the ground in a normal distance or do I need a 5000
foot runway? Would it be underpowered as compared to the O-360 or
would this actually give better performance? Most importantly, what
could I read that actually discusses the engineering questions
involved in answering this question? (By the way, I was an engineering
major for three years before switching majors so I am not afraid of an
engineering level textbook if I have to go that route.)

Thanks.