Well when I thought about it sort of became a bit clearer. I should say that there is actually 3 pre filters, 1 sediment and 2 carbon filters that managed to drop it by 1 TDS.
But when I think about it, they are not there to get rid of "totally dissolved solids". They are there to get rid of solids that are not dissolved, in other words the sort of stuff that will clog up my membranes.
I suppose, the only true way for me to test there effect on DI resin is to run water through some DI resin (250ml) without it being filtered, and then do the same with it being filtered. Testing it at 5 liters, 20, 40 and 60 should give a good set of results with my TDS.
This sounds fun to me, so I will do it but it might take me a week or two to get the bits I need.