Well according to the Noss site chart, 20cc domes at 1000 ft puts out around 156 on compression. So you may get by with those on pump gas.... but with the +4 timing I'm not sure. Maybe someone else with a similar set-up can chime in on the 20's, but I'm sure you'd be safe with the 21's.
And myself, yes I think it would be worth it. I haven't tried any different sizes in domes, but I liked the improvement with the 21's over the stock head.