Cris,
The primary reason for going to a limited slip or locked rear end is that the inside rear tire is slipping during acceleration coming out of a corner. Body roll transfer weight to the outside wheel lightening the inside wheel. An open diff puts the power to the wheel with the least resistance. IE the inside wheel. To avoid wheel slip you must delay acceleration which cost you time. Especially if the corner leads onto a straight. A limited slip or locked diff allows you to put down power sooner. The increased power of both wheels pulling could lead to understeer. (push)
Some things that can be done to over come the understeer are, run heavier springs on the rear, lower rear tire pressure, higher air pressure on the front, run thicker swaybar on the rear, run a thinner swaybar on the front, move weight to the rear, lower the front roll center.
I would start with a heavier rear swaybar. Reducing roll on the rear may be suffcient to eliminate wheel slip, and understeer at the same time. It is the least expensive and has the most potential for improvement.
Chuck