"How much more or less of an effect (if any) would fitting low speed fans to a high flow / large tubing setup in comparison to a low flow / small tubing setup ??"
the reduction in air flow is the same, so the operating point of both will shift (to the right) causing a reduction in dissipation
the dissipation in the low flow rad is less than in the high flow (always), so while both are affected in the same proportion, the absolute magnitude of decrease would be greatest in the high flow rad
try this
and yes, the advantages of a higher flow rate in fact necessiate stronger fans - depending on the rad design of course
- only a single 'type' is shown in the above graph