There are, but if you know the most widely grown peony cultivars, you also know which ones tend to give the highest number of stems.
Check the yearly files (until 2000) for “Production, Postharvest, and Freeze-Drying Evaluations of Fresh-Cut Peonies – Kansas State University” by Karen Gast, they’re in the Hybridizer’s Corner Files section.
These are the results from 2000:
Those files are interesting reading if you’re interested in growing them as cut flowers. They have done testing with long storage, pre-treatments, vase-life testing and so on… Of course, the newest cultivars have not been tested, but then those are probably still too expensive to only use as cut flowers.
You might have a look at some peony showgardens? I don’t know about Sweden, but in Holland there are a few. Green Works has one with some 400-500 cultivars if I recall correctly. R2 Showgarden has one with over 2,000 different cultivars (next year will be their second growing year, so it will be worthwhile to visit from then onwards). Their way of work is similar, a row of some 5-10 plants of each cultivar, all planted at the same distance and depth and year. So it’s easy to compare their qualities then. I’m sure they flower before yours do, so if you would head southwards next year, you can make an appointment (both are non-public, but if you’re planning on growing them as cut flowers, they’ll be happy to show you around, as both are intended to show and sell peony plants).
All the best,