First, there are documented studies showing that known organ donors have a slightly lower survival rate in hospitals than non-donors do. Draw what conclusions from that you will, but some people choose not to be a donor for precisely that reason.
And second, if you have any plans or hopes for cryogenic preservation, that's not particularly compatible with organ donation.
An obvious complication here is that in an opt-in system, people who have more directly faced the possibility of death (extreme sports participants, people with chronic conditions, etc) may be more likely to be donors.
To get numbers with fewer confounding factors, you would need to look at an opt-out system and compare the default opt-in population with a reasonably comparative slice of people who are not automatically donors but have neither intentionally opted out, such as, in the case of the new NHS policy, 'people who have lived in England for less than 12 months before their death'.
> And second, if you have any plans or hopes for cryogenic preservation
If you're planning for cryogenic suspension after death (actually planning for it, not just idly thinking about it), filling out an opt-out form is just a minor footnote compared to the piles of other paperwork needed to set up a legal trust to preserve your body.
I believe they only freeze the head with the brain intact, when one opts for cryonics.
What a loss it would be to come back and be the only one without gills!