As my game resumed yesterday I had all Christmas and Halloween upgrades and could instantly buy nearly 100 prisms from scratch, at only ~1600hc. Decided to reset with a bit more hc because not long before the update I decided to go Elder Pact, but by losing Reindeers so did the Eldeer. Only then, I noticed I could not get any Santa upgrades as well as Christmas cookies and Halloween cookies.
I think that in total my CpS increased relatively but I didn't save a figure pre-reset: I think it would be kind of hard to calculate whether it is useful to keep an endgame with Christmas/Halloween upgrades and save up to 5k hc or reset to benefit more from hcs without Santa upgrades and special cookies. Reindeers, typically, can be left out of the equation as they stop to appear even without a reset.
I think the seasonal upgrades (sans Valentine) themselves add 280% (cookies) + 32% (Santa base multiplier) + 150% (Santa levels) + 50% (Dominion) = 512% multipier. However, this neglects 1.05 times the power of milk, 7% cheaper upgrades and 2% cheaper buildings. Before 5k hc, Reindeers and upgrades do not exist anyway.
The multiplier itself means that resetting with less than +256hc is a loss, and anything above that becomes stronger as it is more; my idea would be that if someone is still in the sub ~4k hc, resetting with at least 400 more is better than grinding out 5k before resetting, but is there a way to think of a definite formula? In terms of gameplay, it's a matter of hours to days at most but a well made decision can make a difference.