The Altezza RS200 is also about 100Kg lighter than the IS200.
As said parts are not difficult to get body, chassis, brakes and suspension parts are the same as the IS200 and the short engine is the same as the MR2 and also some Celicas. The head is however unique to the Tezza.
The reduced weight with higher power and torque makes the Tezza a lot quicker than a stock IS200. A lot of the reduced weight comes from less sound insulation so the Tezza is less refined that the IS.
Fuel consumption is dire on mine, about 23 to the gallon at best, and with a 60 litre tank the range is a lot less.
So you "pays your money and takes your choice" I guess.