I definitely agree with Imperial Seal for Dark Times and anywhere else where there's a decent chance that your search target will be uncounterable. So, in that sense, it's probably underplayed as a sideboard card, if your hate card is both strong enough and against a non-blue deck.
Assuming Vampiric is already in the main, this sideboard snippet is already quite possibly incorrect: 2 Rest in Peace, 1 Karakas. Arguably that should be 1 of each, plus Seal (doubles your access to Karakas while keeping access to RIP roughly the same). To the extent you have access to even more blow-out cards in their respective matchups, the stronger this argument becomes.
Also, keep in mind that Seal is sometimes better against discard strategies than having the hate card itself.
Ignoring issues of "playability" (not like a foil version makes Seal more or less playable), what effect do people think this will have on the price? It's not really clear that Mana Drain as a judge foil had any effect on the retail value of the nonfoil English version, but the demand for Mana Drains is arguably larger and the supply of Imperial Seals was much lower.
From what I understand, there are only 20 being printed for now. One sold on eBay last night for $810 and there are two others listed for $1,000 and $1,500. $1k doesn't seem unreasonable for the low print run and it being the first foiling. I did hear that they will issue two more waves of them to L3's and L2's, similar to how they issued Mana Drains.