If we are talking landing performance (i.e. landing distance) then that's got very little (read nothing) to do with the max rated thrust of the engines, unless we are talking about the go around gradient for the missed approach which I very much doubt would be an issue at Guernsey even at high weights due the the very evident lack of terrain/obstacles.
Crosswind doesn't really affect landing distance unless the flight crew have gone ultra conservative in calculating their landing distance and factored in a tailwind as a worst case scenario to compensate for the potential backing of the wind into a bit of a tailwind component.
Wet runway, now that's a different story and I would imagine that's the real killer at high weights. But as I said earlier an aircraft with the full rated thrust on the engines would have exactly the same issues. So even if Aurigny had purchased the full spec version I'd imagine you'd have been hearing the same story at the time.
Happy to be corrected on any of this by someone who flies the Embraer 195 for Aurigny (all this is coming from an A320 pilot).