Lets get this straight.
There are some items which we recommend genuine is used based on experience (e.g. cam cover gaskets) and others where we recommend aftermarket of a particular type.
In the case of cambelt kits, we have always stated that Gates, Contitech and SKF are good, GM was only used because some years ago they were cheaper than the after market option under trade club which is when its a no brainer (as you get a MUCH better guarantee with the GM items). Since then GM prices have risen more then the aftermarket so there now not as competitive.
On the water pumps, our stance has always been 'every other' cambelt change so effectively 80K miles and 160K miles etc. Again, the GM pumps were cheap via trade club (now silly expensive) BUT, we have seen some aftermarket pumps fail prematurely....what we dont know yet is if the GM ones will fail at say 90K miles if not changed.
Hence its a choice, aftermarket pump which may fail early or aged Gm one which may have little life left......