i agree, 100C is not an ideal temp, but you must not forget that the GPU is a very solid pice of kit, when it gets soldered on the PCB they heat it from above so the solder balls under it(Ball Grid Aray) melts, and solder melts at over 100C(i cant remember the exact temp )
BTW: i dont see any problem with that case's air-flow, if there is not something wrong with the heatpipes it should cool preaty good