The brass housing e1/e3, etc. use similar innards and are rated to 203F. The composite material used in the E5's and E10's is the same stuff car radiator tanks are made of. Those regularly see 230F temps on the inlet nipples and go many years before failure-I suspect Laing is just being cautious.
In any case, since pumps are typically on the supply/upstream side of the heat collector and stratification in the tank makes for lower supply temps to the pump, I think it is unlikely the pumps would ever see sustained temps much above the 140F rating.
Even if used in a radiant heating setup where the pump sees the hottest water, not many are running hotter than 140F supply water.
Many setups do fine with cast iron, but if there is a problem, once you realize it you already have that insulating coating of crud on inner surfaces.
Addding together the benefits of quiet running (you can drown out the sound of my e1'S and e3's with a whisper), the power saving, and the avoidance of corrosion possibility, it was a no-brainer for me.
Craig
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