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I've been thinking about upgrading my outboard A/D to a 24-bit system, but looking in detail at the specifications I see that even relatively high end 24-bit systems (well, high end for my budget) like the Apogee Mini-Me can "only" manage THD plus noise of -94dB. With the usual reckoning of 1 bit = 6dB, surely all I get from recording 24-bit through a device like this is 15 bits of useful data and 9 bits of distortion and noise. Even going up to full pro gear at $4,000 or more per channel it looks like -110dB is about the best that can be achieved, which is still only a little over 18 bits of real signal. Am I missing something here?

If something sounds good, use it.

Of course, but maybe I should have been a bit more specific about by question.

My point is that even though something like the Mini-Me may sound good, if it's only producing 15 bits of signal data, wont't it sound just as good recording at 16 bits rather than 24? Moving to 24 is (I think) going to increase my PCI usage and so I risk the dreaded "PCI Overflow" message more often, and also going to take more disk space too. Am I missing something here, or is 24-bit recording really just snake oil?

it is mostly headroom

The difference between 16 and 24 bits is quite noticable when making music: 24bits gives a dynamic range of 256 times the range of 16bits for only 50% extra of storage space.

I don't use 32bits, as most programs use 32flt which is basically 24bits. On our DSP's audio is processed at 32bit integer, the only program that can record that exactly is VDAT afaik.

There is some difference, technically too. If 24-bit converter has a noisefloor -94dB, thats not pure quanization noise. It's more analog noise from converter amplifiers.
If 16-bit converter has noisefloor -94dB, that's definitely quantization noise.
Analog noise sounds much better, so with 24-bit converter you can go much lower signal levels and still it sounds good, no digital artifacts.
There are no A/D converter yet, which could capture more than 18-20 real bits. But still, I'd see a great benefit to build even 32-bit converter, as then the noise was totally analog and low level signals were clean and pleasing to ear.

The low level parts (close to the noise floor) will have a much higher resolution.

shows that specs can be misleading...

I always use 32bit in Cubase SX because it is noticeably easier to get clarity in a mix, especially complex ones (easier than 16 bit, that is - I haven't really explored 24 bit).....