  We study structure formation in a phenomenological model of modified gravity
which interpolates between LambdaCDM and phenomenological DGP-gravity.
Generalisation of spherical collapse by using the Birkhoff-theorem along with
the modified growth equation shows that the overdensity for spherical collapse
delta_c in these models is significantly lowered compared to LambdaCDM, leading
to enhanced number densities of massive clusters and enhanced cluster merging
rates. We find that delta_c(z) is well fitted by a function of the form
delta_c(z) = a - b\exp(-cz). We examine the sensitivity of PLANCK's and SPT's
Sunyaev-Zel'dovich survey to constrain the modified gravity parameterisation
and find that these experiments can easily distinguish between models with a
cosmological constant and modified gravity, if prior constraints from CMB
temperature and polarisation anisotropies are included.
