This thread got me thinking and gave me the kick in the butt I needed to do something I've been thinking about for a while. I've got about 100' of 1" pex between my owb and my hx with an Alpha 15-55 showing 6 gpm in high. I wanted to bump it up a bit so yesterday ordered another 15-55 and just installed it in series with the existing, one on either side of the hx. The first pump is showing 11 gpm now and new one is at 8 gpm and both are at 43 watt. I don't know how these pumps calculate gpm, but they are both doing the same even though they read different.
Anyways, happy camper, I've got more flow and now have a backup already installed in case one fails. I will just run one on low speed during the summer to keep some circulation going. I'll also start playing with pump speeds to see what happens.