With a max head of 6.5 feet I don't think too much power is going to be a worry.
Most pumps for watercooling have a head of at least 6 feet, many significantly more, and max head is the more interesting figure. To use an analagy, it's a bit like a porche and a truck - the porche goes really fast, but put a load on it and you're screwed. The truck is slow, but you can pull several tons. Your waterblock is that several ton load.
