With TECs 'cold plate' thicknesis pretty critical.
Thermoelectric Cooling module is build of large number (depending on size and cooling power) of thermocouples. Each thermocouple produces module's max dT but has only fraction of module's quoted Qmax. Block side should be as thin as possible (MCW50-T by Swiftech empluys direct module cooling). It does not always hold true since with smaller modules there's not enough surface area to dissipate thermal energy into your coolant at satisfactory rate.
Heat source side is a different matter. Due to size difference (thermal energy density per mm^2) between the module and heat source there's a need for a 'energy funnel' to channel/distribute (depends which way one is looking) thermal gradients. This is where 'cold plate' thickness comes into play.
To cut it short, the smaller the heat source the thicker the cold plate should be (A64 series of CPUs have quite large IHS, which is also of substantial thickness, which allows for thinner cold plate) and vice versa
