A thorough benchmark of DFT functionals without dispersion corrections
There is a benchmark of the GMTKN30 database with and without  the D3 dispersion correction. See Figure 4 here: "Effect of the damping function in dispersion corrected density functional theory"

However, your statement


  I quickly run few calculations including dispersion corrections and found no noticeable difference.


is not supported by this.

Another example is this paper: "Improving intermolecular interactions in DFTB3 using extended polarization from chemical-potential equalization" where we compare PBE to PBE-D3 for 11 different datasets (see the SI). For many dispersion-dominated complexes, DFT (no D3) is qualitatively wrong, and in many other cases, the error is comparable to the mean interaction energy.

In general, I would strongly recommend against DFT without an empirical dispersion correction unless you are only focusing on electronic properties, but then you wouldn't be very interested in GMTKN30 anyway.

Since you say you are using G3/G4 you might also be interested in these recent benchmark papers: 


"Benchmarking Compound Methods (CBS-QB3, CBS-APNO, G3, G4, W1BD) against the Active Thermochemical Tables: Formation Enthalpies of Radicals"
"Accurate reaction barrier heights of pericyclic reactions: Surprisingly large deviations for the CBS-QB3 composite method and their consequences in DFT benchmark studies"