200 whp isn't that hard with the right combination of parts.
a member on another celica board has already dynoed 193whp n/a.
250 (wheel hp) however would be extremely tough and would make for one of the most efficient 1.8's built (asuming you can do it with 1.8 litres)
things to look into having done.
Headwork(oversized titanium valves, custom titanium springs and rocker arms, custom one off camshafts, custom adjustable exhaust cam gear, port and polish)
Custom 4-2-1 race header with no cat
Custom Individual throttle body set up with speed density sensor and custom Intake
air temp sensor (trash the mass airflow meter) with velocity stacks.
Custom EMS(one hasn't been built to run speed density on the 2zz although rumor is AEM
is working on a standalone)
resleve and stroke the engine to 1.93 litres (gt crankshaft)
move the pin further up into the pistion to creat the optimum rod/stroke ratio for high rpm reving (9500+)
custom connecting rods, custom pistons (13-1)
BOAT LOADS OF TUNING, and 100 + octaine gasoline, and I could see getting "close" to 250whp, it wont be streetable, and would need to be towed to every destination.