Growth rates of the axisymmetric instability of elongated plasmas in TCV are measured, in open loop configuration, for a wide variety of plasma shapes with elongations and triangularities in the ranges 1.3 < kappa < 2.2 and -0.3 < delta < 0.6, respectively, and for stability margins covering the range 1.03 < f < 1.60. In addition, the stability of the vertical position control system is investigated experimentally, under closed loop conditions, by varying the feedback gains and by measuring the oscillation frequencies close to the stability limit. The results of these measurements are compared with theoretical calculations, based on a rigid plasma model (RPM) and on a deformable plasma model (DPM). While RPM underestimates the open loop growth rate for large values of delta and overestimates the size of the stable domain in gain space, DPM gives very good agreement with the experimental results.