the 6cm^2 housing on the TD05-16G is not a common size... looks like they picked the sizes to combat lag on the Z32. Their TD06-20G has the same size exhaust housing as used on the Greddy TD05H kit... so lag may not be much worse than the Greddy TD05Hs, with more top end potential.
From what I can see, Pentroof is a tuner that prefers more streetable configs than all-out hp/topspeed tuners like JUN or Escort...