Softer insert (or less packed) is better for side bite as it lets the tire move around to generate grip.
Harder insert (or more packed) is better for forward traction as forces the tire into the surface more by absorbing less of the loading.
TBH I always found I had the wrong inserts mounted in tires when I needed them most so recently I have been only using MrO inserts (45mm rear, 35mm front 25mm 2wd) in their blue compound. That way I have consistency when I swap between sets of tires.
If your gluing up a new set for each meeting / condition change then you can play with inserts much more sensibly but who has the time / money to really do that???
Best stick to a good all round insert like the MrOs and be done with it IMHO.
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