It's all relative to how much vertigo one has. But having done Knife Edge four times and Bondcliff once, I'd say the Knife Edge would definitely prove to be a serious challenge for you, based on what you've said. It's very exhilarating, especially the Chimney Notch which is at the opposite end away from the main peak.
I'd recommend taking another, less imposing route to the summit of Katahdin (like the Saddle). From there, you can test drive the Knife Edge by going out on it a little bit, and then hiking back. If it gives you the willies, you'll know and be able to go back the way you came. If you feel you can handle it, then you'll also know, and can traverse it down.
Best of luck.