  The goal for future neutrino facilities is the determination of the U(e3)
mixing and CP violation in neutrino oscillations. This will require precision
experiments with a very intense neutrino source and energy control. With this
objective in mind, the creation of monochromatic neutrino beams from the
electron capture decay of boosted ions by the SPS of CERN has been proposed. We
discuss the capabilities of such a facility as a function of the energy of the
boost and the baseline for the detector. We conclude that the SPS upgrade to
1000 GeV is crucial to reach a better sensitivity to CP violation iff it is
accompanied by a longer baseline. We compare the physics potential for two
different configurations: I) $\gamma=90$ and $\gamma=195$ (maximum achievable
at present SPS) to Frejus; II) $\gamma=195$ and $\gamma=440$ (maximum
achievable at upgraded SPS) to Canfranc. The main conclusion is that, whereas
the gain in the determination of U(e3) is rather modest, setup II provides much
better sensitivity to CP violation.
