When it's feeling sluggish is the engine still smooth? My Primera always felt that it really came alive properly on beyond 4000rpm. They're not especially torquey engines low down at the best of times.
(TO Prince: Yes the A/C in Primeras is very effective. That, too, can really sap some power.)
Have you considered using a good injector cleaner additive such as Forte Gas Treatment?
This stuff can resolve all manner of issues. If you had a problem with ECU, lambda, MAF etc then the engine malfunction light would almost certainly be on permanently.
EGR (Nissan calls them PCV) valves can also clog and cause performance problems.