I allow 2 instances of LAME to run in the background, (I figure my machine's got the horsepower)

I don't see you benefiting from running too instances of LAME. You computer will just split the CPU power between the two (less some scheduling over head - but lets just ignore that) meaning that each instance of LAME will get the equivalent of an Athlon 800 XP. Meaning that it still take about the same amount of time.
Now if you have a dual or quad CPU computer it would make sense to run as many instance of LAME as you have CPUs. Because as LAME is not a multi-threaded application the other CPUs would lay idle if only one instance was executed.

But then the time taken is not really that big of an issue, is it? Remember these are MP3 to which we are planning on spending a long time listening to - so spending the time on getting them right. Anyway just let the computer encode them overnight - it amazing how much quick a kettle boils when your not watching it!

As far as encoding speed, I use LAME like this : -k -q0 -v --vbr-new -V0 -b192 -B320 -F -ms

As for you choice of LAME settings Dave, you a better man then I. I never looked at the LAME help file till now, and even then I am only fascinated (confused?) by the wide array of options available. But I think you'ld benefit from a visit to r3mix and especially to their forums as these people have spent more time experimenting with the LAME settings than anyone else. Personally I have been more then happy to use the preset options available from LAME, but then maybe you have better ears or better listening equipment then I. /me glances at stock factory speakers.... Of course some would argue the that with the noisy environment of the car it makes little difference - but hey this is what makes the world go 'round.