Training advice for improving my 5K, what is my upside if I can run sun 20 on a 25 mpw base?

To answer your first question I recommend increasing your weekly mileage by 2-4 miles per week to avoid injury. To go from 25 to 45 you should probably target 6-8 weeks.

if you did nothing else losing 20 pounds would improve your 5k time by 15-30 seconds.

For training run 5-6 days a week. You should mix up your pace and do at least one long rub, one tempo run and one speed drill day (400m x 10 at faster than 5k pace) per week.

General rule of thumb for extra weight is 2 seconds per pound for mile. So 15 lbs might be 30s per mile, or 90s for 5k.

For increasing mileage, I would start by getting to the point where you are running 6 days/wk, if you're not already there. For adding mileage, a conservative approach would be to go in 5 mile increments and hold that for 3 weeks followed by a recovery week. So something like 30, 30, 30, 20-25, 35, 35, 35, 25, 40, 40, 40, 25, 45, 45, 45, 25. I like holding a level rather than doing smaller increments every week (ex 30, 32, 34, 36, etc).

As part of your weekly increase, try to also work on increasing your long run a bit. So 9 miles @ 30 mpw, 10 @ 35, 11 @ 40, 12 @ 45. That's just a suggestion. You'll have to work through that and see how you feel with the longer runs. If it's too strenuous, then take a more gradual approach.

18 minutes is certainly doable but will require a change in training.

On the weight it depends a bit on your current body composition. While the charts say you might gain 1-2 seconds per pound per mile, if you are already at a low body fat % the gains will be less than if you are more flabby.

Increasing your mileage will pay the biggest dividends, going from 25 to 45 should net you at least a minute off your current PR.

How hard do you want to work? If you can run sub 20 on 25 mpw at a suboptimal weight you should be able to get to the low 17s or better with a solid training program and dropping 20 pounds. I’d say give yourself 4-6 months to achieve that.