Malmo, I find you posts very insightful. Since I see you have commented multiple times on this thread, hopefully you can respond to this high mileage question that is partly related to tempo-runs.
Do you find that it takes high mileage athletes that are doing slower/moderate base phase runs more races to start seeing benefits? Specifically, does the lack of alot of speed/fast work/tempo mean that a couple more races are needed to "get back in the swing of things" before one can see a big PR from the high mileage?
First of all I don't know where you get the idea that the higher the mileage the slower the running? In fact, you'd most likely find that higher mileage runners are training at faster paces because they are fully training their bodies.
Aerobic training is all at the same pace regardless of how much is being allotted in a 7 day window. But as your body becomes fitter aerobic training zones become faster and faster.
A runner who is not exploiting his body fully at 50 mpw will initially find that the jump to 80 or 90 mpw to be a bit fatiguing. It's not because 80-90mpw should be run slower any slower than 50mpw, it's simply becasue they were undertrained at those lower levels. Not fit. That's not the same as saying that 50-60mpw automatically makes you undertrained, but there's a high probability that you are.
You should be able to spot undertrained runners in competition. They look 'soft'. They don't look very lean. You aren't going to find too many 90-120mpw butterballs out there.
But to address your question as you phrased it, it depends on the race distance and the overall training profiles of the two types. A runner who is training for 1500-3000 with all interval work is generally going to outperform the higher mileage guy in the short-term window, but that difference doesn't last for more than a few weeks. Even with just a few tweaks, an athlete with a larger aerobic base will round into form very quickly.
I remember my first introduction to international competition, during the mid-Summer break in July running with Jos Hermans and Dick Quax in Hermans hometown of Njmegen. Here it was, a 2-3 week break in the middle of the Summer racing season, and these two guys, the best in the world, were putting in 20 mile days. I thought, "hmmmm, these guys know something that I don't" They knew that you just cannot perform well in long distance running without going back and touching on your aerobic base - even if for just a couple of weeks during the racing season.
Go to the training logs of Steve Scotts that have been posted here and you see much of the same thing. Dropping into the 50-60s while traipsing all over Europe, then coming home for 2 or week weeks in the 90s, then back to Europe.