There's nothing available at the moment.
JWT and G-Force engineering both do ECUs for Maximas, but neither has done anything beyond the 1996 model year. (you can swap a `96 into a `97 and it will work, but the gains are smaller) 98+ ECU's with digital odometers haven't been done yet, although it's the newer ECU design (99 or 99.5+ cars) which has had absolutely no work done as far as I know.
Maybe someone can fly to Japan and see if they have anything, because American tuners sure don't.
