I've been using this technique for a while now, though using Nik Dfine instead of Topaz denoise, especially on night aerial images. The technique was thought up by a 17yr old (at the time of posting; it's been published a while) Brazilian K3 shooter. It allows a lot of sharpening without any increase in noise. Worth a look.
It can be very useful but not in all circumstances. I created an action in PS for this and the resultant layer can be left at 100% or reduced for best effect. I don't know if I got it from this guy or someone beat him to it but I have an idea it was Richard who brought the subject up before?
Definitely worth giving a go. It has resurrected a few misses of mine before