Raising the rev limit often requires bigger injectors.

This is because, the higher the revs, the shorter the time you have
to squirt the fuel in so the injectors need to be bigger as they will put
more fuel in for less time.

Generally, unless the injectors are on the limit, you can get away with
about an extra 300 - 500 rpm with changing them providing the
ecu rev limit is removed and the maps are adjusted to suit above
the original rev limit.