I'd like to hear some opinions from experienced marathoners on the importance of weight. I weigh 200 lbs and have run 7 marathons and usually average about 8 minute miles. I want to get faster and I'm not overweight but I'm not "lean" either. 185 would be very lean for me. How much difference would losing 15 lbs make or should I not worry about that and just concentrate on more speed training?

With all other things exactly even, if you weigh less, you will be faster. However, in most cases all other things are not exactly even. I did not train well and weighed 165 four years ago and ran marathons in over 4 hours. I weigh 170 now, sometimes 175 and I've run 3:08, 3:09, and many others between 3:10 and 3:20. Good training and good running fitness are the top things. Lighter weight is gravy if the other parts are improved.

That makes sense although I think in your case you probably added muscle mass whereas in my case I'd be losing fat so it might be a bit more significant for me. So I know from your previous posts you train and run a lot more than I'm going to able to. Given that I have a limited amount of time what type of speed training do you think I would get the most "bang for the buck"? Intervals, sprints and 400s at the track, something else?

It's actually unlikely I added muscle mass since I was a weightlifter for many years before I started running. When I started running I lost a lot of weight after a few months, then I started eating a lot to get it back on.

I'm a bit biased since I don't do any speedwork of any kind other than running races, mostly marathons. However, if the marathon is your focus, the tempo run would be the most beneficial, assuming you have a solid aerobic base. If you don't have solid aerobic base, you'll want to build it before anything more intense. One way to tell what kind of speed work might be beneficial to you would be to look at your race times for short distances through the marathon. For example, enter your most recent time for a one mile through 5k race in to McMillan's running calculator and mcmillanrunning.com and see what times it gives for longer distance races. If your 5k time predicts a faster time for the marathon than you're running, then you either lack endurance and aerobic development or your training is more focused on short distances. If your 5k time predicts a slightly slower marathon time than you've run, then tempo runs would likely be your best addition. If your 5k time predicts a much slower marathon time than you're able to run, then adding some intervals, I'd recommend 800s to 1600s, would likely be effective for you. If you want to run 10ks and half marathons mostly, then tempos and intervals would be a good complement to a good volume of aerobic runs.