Sorry you've been waiting for a reply for so long mick. Probably one of those times when everyone is busy and, I know the site is having problems with email notifications. Hopefully you will be notified of my reply?

Your question is a bit hard to answer. It's not badly written or anything. In fact, I thought it was going to be an easy answer until I stopped to write. (Others probably had the same problem.)

Here's my thoughts/questions...

1. A chest freezer for home-brewers should be seen as just a much better insulated fridge. It should have a controller on it and should not be allowed to drop much below freezing (0°C / 32°F).
2. In your post you say, "if I put 20L of cooling wort into a -20C freezer it should cool it a lot quicker." That's not correct and may even be wrong depending on several things.

I'll stop my points there as I'm sort on time and I think, the main thing to answer is 2 above. Also, even though you gave some good detail in your post, there are still some things that are not clear (e.g. you say you don't chill, but all brewers chill whether it be actively or passively). To save time, I am going to look at some extreme scenarios....

Scenario 1 - Boiling Hot

You drain your just boiled wort into your fermenter (I think that is what you mean by FV) and put it straight into your freezer which is at way below freezing (-20°C / -4 °F). DON'T DO THAT!!! I don't have time to explain why now but someone else could if you ask them to.

Scenario 2 - Too Hot

We don't know how you are chilling but let's assume that your wort is not boiling but is to hot for pitching. In this case, using a fridge/freezer can be a viable option. Whether the "fridge" is at 0 or -22°C actually doesn't make that much difference in how fast you would move from say 30°C to 21°C.

A few more notes...

Oops, I'm out of time now but please give more info on what your current chilling process is (I know it is a passive method but which one?). And, one other thing that strikes me is you mention 21°C / 70 °F for a pitching temp. For me, that is too high for nearly all the styles I know.

So, more info
PP