Like Robjack stated, they are low flow, high head impeller, usually with a matched volute (if you trim the impeller, you need to have a machined volute.
From my experience, the efficiencies are much (if at all) of an improvement over higher specific speed centrifugals that hit the same performance... but it does help with non-overaloading requirements of the driver (shorter curve). Some specs call for a pump to be sized within a percentage of BEP and this type of pump could help.
Sundyne makes straight vane impellers, but they are very small, very high speed designs (15,000 rpm for ex.). Highly machined parts. At the other extreme, I've seen straight vane impellers on March mag drives. I don't expect either to be what your after.
Stepanoff and Karassik don't cover it. You'd likely have to get experience with a manufacturer to learn some of the key aspects. But the basic princicles still appply to all centrifugals...just don't expect high efficiencies.