The effect of "reverse shielding", as I call it, is on the order of one volt of potential difference in light elements where the $n=2$ electrons are the valence electrons.
Let's look at lithium. From this data page the effective nuclear charge on the $1s$ electrons in the neutral atom is 2.691. If we assume that the effective nuclear charge is related to the electronic energy level by
then we infer that the orbital energy level is -98.5 eV for each electron.
Now what is that energy level if we take away the 2s electron? For this we need the electronic energy level of the $1s$ orbital in the lithium ion, which is obtained by averaging the second and third ionization energies of lithium given here. That leads to -99.1 eV per electron. The 2s electron has effectively increased the energy level of each 1s electron by 0.6 eV.
A similar calculation for beryllium gives a difference of about 1.1 eV per $1s$ electron (-184.7 eV from the effective charge on $1s$ in neutral Be vs -185.8 eV from the third and fourth ionization energies after forming the di-cation). The second 2s electron has basically doubled the shielding.
The electron volt differences determined above have some impact on chemical reactions, whose energies are on the order of one electron volt per electron transferred in a redox reaction.