It is very difficult to build a car that is spec’d to maximize performance at a wide variety of circuits.  Especially in Japan, where the variety of race tracks can be very contrasting.  What works at Tsukuba, would most likely underperform at Suzuka, and so on. When Tamura-san made the decision to start modifying his GDB Impreza, he wanted to ensure that the modifications he made had the greatest impact on his times.