Which Tyres?

It will deflate more quickly because the latex is stretched thinner?

Latex 19-23 size tube in a 28c tyre, any predictions?

Depends upon the design, manufacture etc. On our touring bicycles-- where clinchers instead of tubulars get mounted-- I use Pariba Latex 19/23 with 25mm and 26mm tyres. These are handmade inner-tubes and quite variable. Some thinner, some thicker-- the thinner still a much heavier guage than what I've fished out of tubulars.. I'd have no quarms using these with 28mm Continental 4 Seasons..

It will be fine. Maybe fail at the valve seam a year from now... Mind, I only have experience with michelin latex tubes.

Works for me.