However, it's my understanding that the Trim functionality of SSDs writes zeros to the empty space, when files are deleted. So I'm wondering if it would be better to just zero out the drive to begin with. This undermines some of the security of an encrypted drive, but I'm not concered with plausible deniablity, etc.
TRIM doesn't quite work that way. It doesn't just zero pages.
Slack space, if recognized by the drive as such, will often be used as spare area to improve performance. But the mechanics of how and why said space will be used depends on the drive. I don't know enough about the SSDs in the X301 to say whether or not simply leaving a bit of free space when partitioning is good enough.
Personally I wouldn't worry about it. I'd fill the drive with random data to start, set up your LUKS partitions, and think no more of it. Sure, you could have a bit of a long-term performance drop off since LUKS doesn't pass through TRIM (or, depending on the drive, you could not notice any difference.) I did that when I had an X300 and it worked fine. YMMV obviously.
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