Ok, thanks everybody!
So, about the gba version choice: i've chosen this version and not the PS/64 for two main reasons: 1) it already exists and 2) my laptop isnt powerful enough to TAS 32/64/128bits platforms games :)
I could add to this a 3rd reason: challenge! Actually, I've been part of the online TH community for a long time, already done tons of improvisationnal/style vids of this game ( you TASers might enjoy this for example:
http://www.youtube.com/watch?v=o2svrEzghL8 ) so, yeah, i'm good at these last episodes and i know them, and one thing i can tell you is that there wouldnt be enough difference between a TAS speedrun and not TAS speedrun for it to be challenging.
The GBA version has this isometric 3D which makes things way harder to do correctly (for ex: the rail i have to hit after 4:20 in the TAS vid is almost impossible to get because of that approximative gameplay). In real 3D episodes, nothing is impossible to hit when you're a good player. Tools would only help me gain a few seconds but im able to rock the game from A to Z with an untooled single try, whereas on this GBA crappy version, i just couldn't hit the end of the game with a first try single run cause of things like this rail and other places where your eyes aren't enough to know exactly where you will land for sure.
Kind of hard to explain, but i'm sure that those of you who have tried both versions will get my point :)