I would hate if GT5 had fake delays in gear change, simply because changing gear is a skill thing, cars with semi-automatic sequencials change gear instantly, and standard manual mode in GT5 is like that. Sure not all cars shift that way, but the skill of changing gears cannot be put onto a game pad, or flappy paddles, so all you can get is what is basicly artificial delays, taking control out of the hands of the player. The only thing it will do is make sequencial shifting much much slower than clutch shifting, which in a game that is meant to be fair for gamepad users is totally out of the question.
There are some compromises that are the lesser of 2 evils, i really would hate if GT5s sequencial mode went down the route of the game deciding how fast you would change gear in a manual.
Not the best example, being a racing car, but the Lancia Delta vid shows perfectly what is possible with a full manual, and that certaintly wasnt slow. Not everybody needs to spend 8 years between changing gears, like in the TDU2 vid.