Any downside to running an all-out mile (or 2 mile/5k) during base?

I don't see want down sides.

No, as long as if you do worse than expected it doesn't discourage you so much that you quit.

I assume you are doing a "classic" base, where you aren't doing any workouts at all. Just easy runs and maybe a couple strides?

If you haven't run a race in 6 years and run an all out mile after only that sort of base training you could get injured. I knew someone who ran a mile all out in the middle of base. He ran a 4:19 (about 8 seconds off his PR) and was super excited and was looking forward to how fast he would be in a few months after proper workouts, races etc. The next day wasn't so great and he could barely walk. He ended up having to get surgery on his achilles and has never been the same.

I would at least do some sort of not too stressful workout a week before. Something like 4x400. That won't really hurt the overall base phase principle and will get your muscles/tendons used to a race somewhat.

But even better would be to run 2 miles at the perceived effort of a Tempo and then extrapolate from that.

Was your friend doing what you call a ‘classic’ base before the all-out mile? As in only easy runs and strides?

So far I’ve done fartlek workouts around 3 mile pace, as well as speed work such as flying 150s.

I agree, there's a potential of you getting injured, I had something similar happen to me last year. I was only 2-5 weeks into my base training and raced 800m in between that and ended up pulling my quad-muscles :(

I would suggest you do at least 3-5 speed workouts 2-3 weeks before racing an all-out mile.

cheers!

Go for it and have fun!

I don't see any problem with doing a hard effort in training to assess your fitness. You wouldn't even have to empty the tank. If you do a steady 5k without pushing it 100% could still gauge what level you at at off the effort.