Flow should increase not decrease with a calorifier (Heat exchanger)... Check that the cold mains inlet to the appliance has been re-opened all the way, there is no flow restrictor in these appliances to my knowledge, so flow rate should only be effected by the isolation cock...
If this doesn't work, all I can think of is that possibly an 'O'Ring or Washer is restricting the waterway... This would only be possible if the old seals were left on the appliance, otherwise you'd be having huge waterleaks by now if the new seal kit wasn't fitted properly...
Let us know how you get on.
ccMods
August 2006