Finding new or modified rows since snapshot

My goal is to select rows that are new or have been modified since a given
snapshot. I am doing it like this:

SELECT * FROM my_table WHERE NOT
txid_visible_in_snapshot(xmin::TEXT::BIGINT,
'123456:123456:'::TXID_SNAPSHOT);

On one hand, it seems to me that the txid_visible_in_snapshot function was
taylor-made for what I am trying to accomplish. On the other hand, it seems
wrong because the first argument of txid_visible_in_snapshot is BIGINT, and
I am trying to coerce a xmin which is of type XID into this argument.

So I have three questions:
1) Is this a good way to accomplish my goal (keep in mind I have read-only
access to the db)?
2) Will it fail when xmin wraps around at 2^32 or is
txid_visible_in_snapshot smart about it?
3) Is it going to be performant (xmin is not indexed, is it)?

Thanks,
Meel