Is there a universally recognised 'stiffness scale' to enable purchasers to make comparisons between various skates?

No. They put recommended skills by boot type for each manufacturer, but each sees it a little different. You have to read about what the boot is designed to do (Basic, singles, Axels, doubles/triples) and determine where you fit in the scale.

FWIW I strongly feel that I suffered for years because I went with the recommended boot stiffness on each pair and I felt that I was skating in ski boots. I finally went for the hinged Jacksons and it wasn't a moment too soon. Now I can finally bend my ankles.
Lyle

It's not just what jumps you are doing, either. It's very important to factor in body weight and how aggressively you skate.

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"You don't have to put an age limit on your dreams." - Dara Torres, 41, after her 2nd medal at the 2008 Olympics

Indeed. I don't do anything but some lousy singles yet but had broken down a pair of Graf Edmonton Specials (stiffest boot Graf makes unless you get the extra-stiff model) in less than a year. :/

But now I have medium stiffness Klingbeils and I wonder sometimes if they're not just a tad too stiff as I can't get down into sit spins very well (but maybe I'm just being a bad skater grasping for excuses...). In any case, the "medium" stiffness Klingbeils are stiffer than the high-end Grafs were, and I've heard that SP Teris are stiffer still.

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Casey Allen Shobe | http://casey.shobe.info
"What matters is not experience per se but 'effortful study'."
"At first, dreams seem impossible, then improbable, and eventually inevitable" ~ Christopher Reeve

It kind of depends on which SP-Teri boot you're comparing to the other bootmakers. Like most bootmakers, there are several levels of stiffness... so nearly any one of them COULD over-or-under-boot you. That's why it's best to talk to the person you're buying the boots from about what elements you're working on and hope to be working on in the near future, along with mentioning how many hours you skate each day and how many times you skate each week, and they can probably "guesstimate" your weight when they see you... all to help you find the perfect stiffness for your skating.