Nape,
Looking at the Ackley Improved numbers there is a substantial increase in velocity and case capacity increase. The Gibbs version has even more case capacity increase. While I don't have the exact numbers, Quick Load can esitmate what the velocity increase would be.
Alot would depend on sutable powder choices. H4831SC, IMR7828SC, H1000, RL22, and H4350 are all good powders to start with. H4831 and RL-22 are excellent in the parent case, so they would be a good place to start. Highest velocitys should be with RL-22.
I think a 28" bbl would serve this round nicely to get a high rate of burn with a slower powder.