Hey Xaerox,
Thanks for the questions!
1. We build Lemans from the ground up specifically and only for the Dreamcast, the rendering engine, physics simulation, assets, all of it. A bit of a work of passion really and I think the approach was much like how Sega itself might build a first party game. I actually brought the game to Sega Japan and they were extremely surprised about some of the things the game was doing! They took me around all of the Sega AM groups to get their feedback and actually wanted to publish it worldwide as a 1st party Sega title ;-)
2. It's perhaps a myth for the final shipping version of the game! The game uses a sustained 50,000 polygons per frame + effects at 30FPS. It renders a pretty constant load balanced 25,000 for cars and 25,000 for the circuit per frame. So that's really 1.5M polygons per second, call it close to 2 with all effects lol. However, the graphics engine is very optimized and can do 4 million perhaps 5 million polygons per second! Lemans had the unique problem of having 25 cars on track at the same time. Every car has the same sophisticated physics simulation as the player car, as well as an AI driver and associated audio - the races are authentic, and all cars behave with the same physics characteristics as the player has. This uses a lot of CPU resources, compromising how much of the CPU can be dedicated to 3D transforms and feeding the GPU with vertices! There is an unreleased early version of the game that we showed at E3 that has one finished track and 8 finished cars on track that runs with the same polygon count at a sustained 60fps! In that version with 8 cars on track it does 50,000 polygons / frame at 60FPS = 3M polygons / second.
3. We would love to have made Grand Prix Challenge for Dreamcast as well. It uses a similar in-house engine, but this time optimized ground up for PS2. The Dreamcast could run it at 30FPS with some changes. GPC uses 2-3 times the polygon counts on PS2 at what looks like double the frame rate. However, it is an illusion and is cheating! For the longest time GPC was stuck at 30FPS on PS2, however late in development we discovered a secret that literally doubles the apparent frame rate! Essentially, we are doing something similar to DLSS3 on PS2 and I'm amazed it's taken this long for something like DLSS3 to appear! GPC runs at 30FPS, however it generates the next frame an in in between interpolated image from the prior frame to deliver 60FPS in a 30FPS game
Transformers Armada uses the same trick and in hindsight it might have been possible to do that on a Dreamcast as well! In any case if GPC were ever made for Dreamcast it would have less polygons and better image quality than the PS2 game.
4. It's awesome to see people still trying to do things like on Dreamcast! If the graphics engine and assets were rebuilt from the ground up for Dreamcast there is no reason you couldn't do a pretty cool version of GTA III for it. However, given the original game uses Renderware that ran quite badly on PS2 and is very slow in first place it'll be a challenge to make it run beautifully on Dreamcast. Good luck with it though
Cheers to you in Colombia!