In theory, an air breathing rocket engine could have an Isp in the neighborhood of 3000 seconds or so. That's a tremendous improvement over conventional rockets. The SSMEs Isp is somewhere around 450 seconds.

However, I wonder how well an air breathing rocket could fit most launch profiles. If you launch vertically, then you're rapidly accelerating and climbing away from the oxygen you need for combustion. You probably couldn't sustain combustion much above 150,000 feet (about 30 miles). If you try to flatten your trajectory to get a bigger boost from the engine, then you're probably increasing the thermal loads because you're staying in thicker air for longer periods.

Perhaps a better approach would be to use a TSTO design with a mother ship (similar to DARPA's SCAMP concept) to carry a booster with the payload as high and as fast as possible. SCAMP is taking about launching the booster at around Mach 3 and over 100,000 feet high. An air breathing rocket engine might be able to get it a lot faster and perhaps higher before release, increasing the payload to orbit.