try using MK's garage editor to up the HP of your cars with a power multiplier, or something like that Famine would be the guy to talk to in that respect
you can change the boost characteristics in GT1. Since turbo lag was a major factor in that game, you could choose for a quick response but less hp or slow response but big power.
yeh it would be a good addition to GT4, also the latest sega gt had boost changes to, but if ya maxed out the prssure the turbo would blow before 10 races lol