You always get used to the power.
I had the smaller turbos on there, for a street car that i took to hill climbs/motorkhana's and so on on the weekends it was the best setup.
Putting -5s on they are laggier, and 1st/2nd is pointless as you really need to be 140km/h++ before the -5s come into their own. The only reason -5s feel faster is because they take longer to spool and then hit harder with the torque later on. Yeah they make more power so at a track like Philip Island they might well give you benefit - but go to Sandown/Winton the smaller ones definitely shine.
Plus you'd still need to do transfer case, front/rear diff, suspension. They make the biggest difference dollar vs gains IMO over power/rebuild - just there is 10k on its own. So add in another 15k for the motor and everything and you've done 25k there. Hence why my GTR was such a bargin really @ 30k, and sold easily
And his MPH @ the track showed he didn't have anymore than 300rwkw at the time. So take the result with a massive grain of salt.