It is my understanding that Nikon has been working to put significantly stronger IR filtering in front of the sensor in recent bodies, to improve IQ. In these bodies just using an external filter should be problematic, Nikon does not want them to respond to IR. Just a bit of reading on the internet indicated the D70 was the best model for IR, out of the box. It was made before the trend to stronger IR filters in front of the sensor.
It also implies that for the D300s, if you get it modified to be an IR camera, it will work much better than just using an external filter. The mod removes the existing filter and replaces it with one made to pass IR. This is probably all better explained on the web site of the IR filter manufacturer.