The water would have to be over 120* before it even effected the cooling of the engine IMO, there is so much cooling available that you will probably still have to throttle it to get it to warm up, think about your car, the water leaving the radiator on a hot day is like 150* and the engine still runs around 190*, I would run the water through the manifolds first. Seriously doubt it would cause any problems..
On the flip side of that, hotter exhaust pipes flow better, so I guess its a catch 22...
