I've got a save file which has everything done toward 100% except the shooting range (forgot to do it the second time for the fast reload unlockable). No matter which cheats are entered before going in and completing it, 100% is reached.
Absolute rubbish. I had to use the "slow down time" cheat otherwise I never would have won all the races or got the "Driver" mission. I could never get to grips with the driving mechanics in the GTA PS2 games...too arcadey and my reflexes aren't all that great. Going slo-mo was the only way I could avoid smashing the vehicle into anything.
I only cheat code I remember in which I received the "Warning!..." message was the "Spawn a Rhino" cheat (I initially used it for "Death Row" mission before I realized using a helicopter was just fine). When I play this game nowadays, the only cheat I use is the "Blow Up Cars" cheat (for the easy money; I'm just not big into the side jobs, and I like to buy The Malibu as early as possible. Also, I like Tommy to have some fold'n money left over after purchasing all the assets).
The only times I cheated was the doing the health cheat during the vigilante, paramedic, and firefighter missions to keep the vehicle from getting damaged too much and blowing up and during the last story mission. Even with 150 health and 150 armor that mission is a pain in the ass.
I got 100% though so like another person said "absolute rubbish".
I cheat, in a way, on the final mission: instead of getting into a gunfight with Sonny and his henchmen, I just run him over with the Infermus. That way also the allows the possibility for two Tommy characters onscreen during the cutscene and the Infermus hanging out in the hallway, which amuses me.