This will probably just end up being a superficial difference. It would be the same as X1 one make racing, except the visuals. Making a ultimate race car can be hard enough, but then trying to make a dozen that are all different but equal would be even harder.
And what is so bad about one make racing anyway? Yes the X1 can only be beat by another X1, but that is still racing. Dodge, Ferrari, Toyota, whoever all have one make races in reality, why can't the X1?
That would give you a F1 killer, not a LMP competitor. Without the fan, the X1 still have wings that are better than F1 cars and a full ground effect underbody. You can't make the X1 as slow as LMP's unless it's towing a dump truck.
However, with weight and power adjustments in GT5, you can turn the X1 into a completely different car. It's fun to drive in unrestricted and restricted form.