Oh, when you said tea bread, I thought you meant a quick bread. Yeast bread? You've got a LOT of leeway there. As long as you don't 'cook' your yeast and kill it, the sky's the limit.
For example, the roll recipe I've developed after years of experimenting (to try to duplicate my dad's wonderful rolls), uses 1/2 cup sugar, to only about 3-4 cups flour.
More sugar in a yeast recipe is actually a good thing, because, as you probably know, it's food for the yeast and helps it grow well.
I'd say you could easily go up to 1/2-2/3 cup sugar for those recipes. You might also consider upping the butter a little on that second recipe--that also helps make a richer, more flavorful, more tender (cake-like) dough. I'd use a full stick for each recipe (you culd always cut back a hair on the liquid to make up for the extra butter)
If I were experimenting with those, I'd start by doing 1/2 cup sugar in the first recipe; and using a whole stick of butter and 3/4 cup sugar in the second.
Good luck.