Loopback Latency of an Audio CODEC?
Didn't you just specify the answer?
You said spacing of &quot;a couple inches&quot;.  Sound is roughly 1ms per foot.  Let's assume 4 inches which means roughly 400us (I'm being VERY loose to make the math easy).
At 48KHz, samples are roughly every 20us--so you get 20 samples (400us/20us) to stay within your latency budget.
If you run that up to 192KHz, you get 80 samples.
The codecs are probably close to 1 sample of latency on DAC and ADC if you turn off on-board DSP processing.  Nobody actually tries to delay things (unless you have on-board processing and ask them to), but you have to have a full sample in order to transmit/receive on the digital bus.
As for vector instructions, even if you only had 1 current time sample to work with, you've got lots of DSP algorithms which work with quite a few past time samples so SIMD vector instructions would still be a win.
I would also caution you about being too stringent in your audio specifications.  Most people have ... quite imperfect hearing.  I have created audio systems where there was a variable 5-8% pitch shift (for delay compensation reasons in really bad cases).  It was painful for me to hear, but the vast majority of people didn't even notice even after I pointed it out.
I recommend that you set your specifications to the human perception average and allow tweaks for those who are outside the norm.
Hope this helps.