If I'm browsing for races I'll always check the runbritain race listings because you can be 99% sure that they are UKA licensed (the other 1% being 'Pending' licenses which never get completed, like my first and probably only ever race win in a small local race which was supposed to be licensed but they never finished the paperwork, but that's another story).
As far as I'm concerned ARC races are on a par with fun runs, and I'd prefer it if the clubs that organised them would pick their toys up.
I prefer my results - particularly PBs - to be on Power of 10 / Runbritain. I'll occasionally do a non UKA event if there's a particular location or charity that appeals to me, or friends keen to run it, when that happens then I log in and record the result in the 'About' section on my Po10 profile so that I can still see all results in the same place. As per literatin I wouldn't treat one of these as a goal race though.
2:53 is a rather fast 800m rep time for somebody targeting a 3:15 marathon - I suspect you're doing too few reps and/or too long a recovery between to get the benefits. I built up to 8x800m averaging 3:00 when marathon training (race result was 3:04:3x).
7:50 min/mile is also too fast for long runs for a 3:15 marathoner, and 26km is too short for a long run unless you're quite early on in a plan or it's a cutback week. My long runs were ~8 min/mile with about one in three of my long runs having a fast finish.
And 7 min/mile pace is bonkers pace for an 'easy' run at your level. Hopefully the treadmill is wrong and you are actually doing around 8 min/mile pace, otherwise I fear you will burn out by race day.
Thanks for the compliments, but really the first chunk of improvement is the easy bit once the initial motivation is there. Now on the shallower tail of the diminishing returns curve, trying to cut another 10-20 seconds off of the 5K PB takes much harder work. There are people with less natural speed putting in a lot more effort than me, and there are people I know with more natural speed and talent than me who continue to improve rapidly and leapfrog me after I've started to plateau. It's hard to see where you are at since you don't seem to have any race times to judge from?
3:57 to 3:15 looks like a large difference, but the last September marathon sounds so badly planned/prepared that it's difficult to read much from it.
Do you have any recent shorter race times as a guide to support your 3:15 goal?
Saying that you "exceed paces from training plans" can be a good sign if you're referring to hitting faster interval session or tempo run paces, but if you're running all or most of your long runs too fast then that will have a detrimental effect for most people.
In answer to your actual question, my largest improvements, as is the case with most people, were when I first started regular training and racing. In 9 months I went from 24min 5K to 18:19 5K, and from 49:40 10K to 39:30 10K. But in the two years since I've only shaved a few seconds off of the 5K time and a couple of minutes off of the 10K time.
I try and hit 'race weight' for important races but it's not a weight I can sustain long term and be healthy and train well.
I do this too, maybe 2-3 times a year I'll bother to drop the extra ~2KG above racing weight, that I tend to carry when training normally and treating myself to some red wine and chocolate when I feel like it.
This seemed to work for my marathon and HM PBs. However my fastest of four 10K races so far this year was actually at my heaviest - mainly because it was the best weather of the lot. ie once fairly lean and fit, all the other variables on race day are much more likely to make a noticeable difference than 1-2KG either side, so whilst it might help you'll never know for sure.
But Tom you're talking about losing 16lbs in 3 weeks, which is equivalent to 56000 calories in 21 days, so a calorie deficit of around 2600 calories per day. That's not possible to do healthily, especially for someone already at a healthy weight. The 5 pounds lost already is probably mostly your dwindling glycogen reserves being used up and the associated water weight that will have been shed with it, it's extremely unlikely that you would have lost 5 pounds of fat within a week unless either by starving yourself or by eating normally and running a 20miler every day!