Is an external SSD drive 550 mb/s fast enough for working with FCP projects ?

Even a cheap USB 3.0 drive works fine for 1080p and 4K editing.

I have just one SSD and several USB 3.0 HDDs.

I don't use the SSD any more for editing as I find the USB 3.0 HDDs are more than fast enough even for 4K

Storage wise a USB 3.0 gives more than 5 times as much for your money.

As Ian said, you could make do with even a HD.

Of course, if you can, you should use an SSD for editing, and copy stuff off to cheaper hard drives for backup and archiving.

And yes, if even a 150MBps hard drive can do it, the SSD can.

Also, I don't recall you mentioning your mac model. If your mac has USB-C, it can take advantage of the faster SSD, but I doubt that you will notice the difference, anyway.

I can offer a bit of personal experience: I have two Crucial SSD that can reach about 1GBps on USB-C, but of course will top out at just under 500MBps on a USB-A port. I have used them on my older 2014 MBP, and on my newer 16" 2019 MBP. They work fine on both. In each case, the SSD were amply capable of supporting work in 4K, even in some cases using multicam. They are not a problem, at all.

An external SSD works FINE.