That's a bit double-edged. There are Horrors powerfull enough to exist in the current magic level, like Yrsgranthe, Bone Crown, and others who
already tried to get here. On the other hand, there should be also less powerfull Horrors, who find the current magic level enough... By reading the ED introductory history from the rulebook you can see many minor Horrors manifested quite early. And powerfull ones too - like the one who was behind the fall of Scytha - a dwarven kingdom, in which all newborn died in a few weeks, like something was draining their lifeforce... That was quite early, so magic level wasn't very high, and tell me you don't think a powerfull Horror was behind all that.
If you want to introduce Horrors in the game, do so with minor ones. There are Horrors, who you'd consider weak in the physical sense, that have enough brains and manipulative power to be a serious threat to the group. Now, what if they are hired to catch one?
The way I figure it, the mana level is lower so any Horror that could reach through wouldn't be AS powerful. Spirit pacts (which even RAW can suck for the PC) seem to fill the niche. They're just more deliberately abusive with their end of the bargain and can provide some fascinatingly evil powers.
Well, not really. Whatever the Horror promises is either a lie (no cake), or will be twisted in such a cruel and painfull way, that it will probably drive you insane. There is no 'pact' in Marking. Just an alien, evil entity making the Marked one his bitch. Period.