A substituent stabilises a carbocation by $10\mathrm{kcal}\mathrm{mo}{\mathrm{l}}^{-1}$ relative to the unsubstituted cation. The transition state of the (endothermic) ionisation step that forms this cation is stabilised by

The transition state of an endothermic step is late and has partly developed the positive charge, so it feels most of the stabilisation. It is however not a fully formed cation. So its stabilisation is a large fraction of 10 kcal/mol, but less than 10. It would be zero only for a transition state identical to the reactant and exactly 10 only for one identical to the cation.