Does anyone have any insight on which is the best for 5K performance, running more miles with a majority of miles being done at an easy pace (I have read 80 to 90%) or running less total miles with a greater percentage being focused on speed and stamina.

For most people, more miles will actually be more effective in improving 5k time. Don't get me wrong -- to truly peak, you need both, but the fact is that even a short race like 5k is mostly aerobic. More base miles will build your aerobic capacity more completely than low miles with speed. Speedwork is icing on the cake, but most people need to focus on the cake first. When I was in HS, my coach gave me an off-season training program for the winter between cross-country and track. I remember he specifically wrote "no speedwork until you get up to 40~45 miles per week." His point was to emphasize a strong aerobic base before focusing on speed.

Is there some ceiling for mileage when it comes to 5K racing and performance? Is there any point where runners would begin to see diminishing returns in performance?

I think there is, but it differs from runner to runner. This may seem a little contradictory to what I said about it being a mostly aerobic event, but you do need to have a certain amount of snap in your legs to bust out a good 5k. If you pile on too many miles, you might feel a little dead-legged when race day comes. That threshold isn't likely in the 35~40mpw where you are (and where I hope to be in a about a month). If you're serious about 5k racing and are doing serious speed workouts (hard intervals, fast tempos), and you're trying to peak for a certain race or series of them, it's probably a good idea to back off a little on the miles during that last 8 weeks of sharpening approaching the peak race. That's assuming you've built up a real solid base prior to that sharpening phase.

I just PR'd in my 5K yesterday by about 55sec, and I've done almost no speedwork for a month, so apparently getting my mileage up has worked for me -- dunno how that applies to anyone else, but I'm certainly happy