I would think that under normal circumstances, you can take improved familiar at level 1. The "sufficiently high level (see below)" requirement is saying you must be high enough level to pick one of the familiars on this list. But the Feat does not force you to pick a familiar from that list, it says:" When choosing a familiar, the creatures listed here are also available to you." (emphasis mine)
When you take this feat, you gain the option to take a familiar off the improved list, provided you have a sufficiently high level for the familiar you pick, this requirement is a variable that changes depending on what you choose. If you pick an Arbiter, you must be 7th level or higher, if you pick a Raven there is no minimum level.
I would also say, if you disagree with my last post, and decide or rule that you cannot take it at first level, would you allow a Winter Witch to take it at first level?
One of the abilities of the Winter Witch states:
Winter witches must choose a familiar that is native to the frozen north, even when they themselves operate in other regions. Traditionally, this limits winter witch familiar choices to bat, cat, fox, hawk, owl, rat, raven, or weasel. A winter witch who gains the Improved Familiar feat can select any familiar she desires, save for familiars with the fire subtype.
So would you allow a Winter Witch to take Improved Familiar at level 1 to increase their options of a starting familiar?