Finding new or modified rows since snapshot

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