T-VSV, or Turbo Vacuum Switching Valve is the name of a system which is fitted on all GT-Fours. This valve is the reason why the boost in the first two gears is lower (ST185: 50 kPa) than in the rest of the gears (ST185: 65-70 kPa). To get the same boost in all gears, you can disconnect the T-VSV. However, without fitting a boost controller, the boost will be 50 instead of 70 kPa in all gears.
HKS New EVC boostcontroller
How to disconnect the T-VSV system
Modifications - Engine