Are there any cars in GT2 that are fully customizable, being able to be tuned with every option? Let me know about any weird tuning features for any cars!
Technically, no: Only a handful of cars can get the Yaw Contol option; and none of those "yaw" cars can get the NA Tune-Up 1-3 instead of an available Turbo Stage 1-4 option.
Most road-going cars can get a lot of different upgrades, only a few can get only power or weight reduction as an option, if that makes any sence. Just about every racing car, save the GT40 (that is, cars with an [R] to the left of them without performing the race modification) can't have any other modification other than the TCS and ASM.
Messa round with a few cars, and you'll see how it all works.
Yeah, and only a few cars can get the displacement increase--for which there's no replacement. This is kinda bogus, though. Just about any car's block can be bored out IRL, especially all the old American ones.