I don´t think it should have more cc, then it would´nt be a 400. A ZXR400 with a ZZR600engine in it is nice, yes - but not a 400.
The standard ZXR400SP is as near ultimate a 400 can be in my opinion. Change the dampers and you´re all set - no one with a 400 is even near.
Perhaps a carbonfairingset and carbonwheels to loose some weight.
Thats my dream of an ultimate ZXR400.
I have a slightly tuned ZXR400SP wich deliveres 74BHP at the rear wheel(original exhaust). With the carbonwheels/fairing and a full titaniumsystem(if I can find one) I think it would be the blast!!
Picture of a ZXR400SP with a Nassertsystem. A bit wrecked though - not my bike.