Nismo's 340hp R-tune Z was announced yesterday at the 2003 Tokyo Auto Salon. For now it's a development prototype "for circuit use" but it's a good sign of things to come.
Nismo has equipped this car with full R-tune parts catalogue (ECU, suspension, full exhaust system, bonnet, etc) - aside from previous sporty S-tune Z33.
In case you were wondering, the 280hp limit only applies to primary manfr. which Nismo is not. It's similiar to how the Germans have self imposed limits like 155 top speed which do not apply to firms like RUF, Brabus, Gemballa, Hartge, etc. although in Germany they are considered manfr. too.